Google Search Disruptions in China

Before Google’s announcement to exit China, Google China used to censor organic search results and prevent certain search results (that was considered inappropriate according to China’s Internet laws before March 2010) from showing up on Google when users searched from within Mainland China.

Google’s decision was to redirect all China-based search users from google.cn to google.com.hk. The story continues.

A large number of Google searches from within Mainland China have been disrupted and/or blocked temporarily due to many more “sensitive” keywords in recent months. This may have caused Google’s web search users to believe Google’s web search has technical issues from within Mainland.

While these disruptions are outside Google’s control and unrelated to our technology, we apologize for the inconvenience.

Google shows this isn’t the case of technical issue from Google, but is due to certain keywords being blocked from within Mainland China.

In order to figure out which keywords are causing problems, a team of engineers in the US reviewed the 350,000 most popular search queries in China. In their research, they looked at multiple signals to identify the disruptive queries, and from there they identified specific terms at the root of the issue. We’ve observed that many of the terms triggering error messages are simple everyday Chinese characters, which can have different meanings in different contexts.

Google provides 2 options with messages to search users when they have encountered the interruption issue: 1. Edit Search Term, 2. Search Anyway.

We’ve observed that searching for [SOME CHINESE CHARACTERS] in Mainland China may temporarily break your connection to Google. This interruption is outside Google’s control – 1. Edit Search Term, 2. Search Anyway.

    Google Search Disruptions in China

It may not be possible for Google to ease the relationship with the Mainland Chinese government, Google’s best option for now is to have user experience improved for all web searches from within China.

Posted on June 2, 2012
Category: Google China | Leave a Comment

Coding vs Blogging, Spend Time on Programming

Many times in life you arrive at a point where you need to decide one of the two:

  • Move on to a chase different goals
  • Learn a new skill

I previously explained the purposes of my blog (or blogs):

Hours in Coding

I have been picking up a bit of coding during my spare time in 2011 mainly due to self-interest, while I have been updating my blog (Gordon Choi’s Blog) and posting to Clickz Asia.

So I estimated the number of hours I have spent on coding per year from 1998 to 2011 – A total of 6211 hours (excluding all hours I did spent before 1998):

    Hours Spent on Coding

Then I looked at the number of blog posts which I have published on Gordon Choi’s Blog, my Chinese blog and Clickz Asia from 2006 to 2011:

    Number of Blog Posts 2006-2011

Finally, the correlation between coding and blogging is when I spent large amount of time on blogging, then I did not spend much or almost any time at all on coding. Year 2011 was an exception which was when I re-picked up coding while I have still published 146 blog posts in total.

  • In the very early years (1998-2001), my career objective was to become a full-time programmer.
  • A shift in career path happened started in 2002. For about 9 years and a bit more (2002-2010) I have been in the career of the search marketer, web analyst roles.
  • In 2011, I tried to re-gain the ability in programming which I believe would make a large part of my life much easier and give me a longer stretch into the unknown future.
    Coding vs Blogging

Programming Languages

Programming languages that I have ever used including Pascal, C, C++, Cobol, and Python. My criteria for inclusion are:

  • I didn’t include HTML as many programmers believe HTML isn’t a programming language at all.
  • I did at least successful code several algorithms in all these programming languages, and for some programming languages I did even code countless number of lines and completed some applications that other people could use to make their lives easier.

Of all the programming languages that I have done coding for, they are in one of the stages:

  • I have very unlimited experience/knowledge due to lack of practice.
  • I am comfortable to a certain level but never have another opportunity to practice and improve.
  • I am comfortable to a certain level and have given up.
  • I am comfortable to a certain level and am still pretty much using it in coding.

Definition of Coding (or Computer Programming)

Wikipedia defines “coding” or “computer programming” as below:

Computer programming (or coding) is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a set of instructions that computers use to perform specific operations or to exhibit desired behaviors. The process of writing source code often requires expertise in many different subjects, including knowledge of the application domain, specialized algorithms and formal logic. Within software engineering, programming (the implementation) is regarded as one phase in a software development process.

Multiple Skills: Search + Analytics + Coding

I was once programming in full-time (which was many years ago), but I have stopped being one. Even I have restarted the habits of writing some codes that probably have made my life a bit easier since last year, it still doesn’t mean that I’m good at coding at all. In fact, I still have a lot to learn.

It’s great to have skills and years of experiences in PPC, SEO, blogging/wordpress, online marketing in general, web analytics, social media and more. Life can even be better when you can write codes in one or several programming languages. You’ll never know what you can do with it until you try hard!

Posted on May 22, 2012
Category: Coding | 1 Comment

Google Panda Algorithm

Google’s Panda Algorithm was first rolled out in February 2011 to hit large content farms by lowering the rankings of web pages with high bounce rates and low quality external links.

Google Panda Algorithm to Affect Content Farm Sites

When the Panda Algorithm lowers a website’s ranking in Google’s search engine results pages (SERP), the website’s visits and page views from Google SEO will drop as a direct result. Sites / companies that suffered hard because of Google’s Panda Algorithm include:

  • About.com
  • Some of Yahoo’s content pages
  • Demand Media

People working on the following roles within the Internet industry should pay attention to Google’s Panda Algorithm updates:

  • Professionals whose role is to manage search engine optimization (SEO) for websites that have large quantity of text content.
  • Webmasters who own and manage multiple websites with multilingual content.

Google Panda Algorithm Updates

  • Panda 1.0 – The first version of Google’s Panda Algorithm was rolled out in February 2011 that hit content farms which caused irrelevant organic search results to show up in Google and affected 12% of searches in the US.
  • Panda 2.0 – Released in April 2011 to all English language search queries worldwide including: British English (google.co.uk), Australian English (google.com.au) and English language search results (e.g. google.fr, google.com.hk).
  • Panda 2.1, 2.2, 2.3 – Several minor algorithm updates in May, June and July 2011.
  • Panda 2.4 – Released in August 2011 for English language search queries globally and non-English language search queries except for Chinese, Japanese, and Korean, and affected 6-9% of search queries.
  • Panda 2.5 and Panda Flux – Released from September to November 2011 and affected 2% of search queries.
  • Panda 3.1 – After Panda 2.5, it went straight to Panda 3.1 (released in November 2011) without an official Panda 3.0 algorithm update. Since Panda 3.1, algorithm updates have entered a period of Panda Flux where updates started to happen more frequently and were relatively minor.
  • Panda 3.2 – Released in January 2012 and this Panda data update was confirmed by Google.
  • Panda 3.3 – Released in February 2012 as a Panda Flux.
  • Panda 3.4 – Released in March 2012 and affected 1.6% of search queries.

SEOmoz provides a list of all Google Algorithm changes that includes the incremental Panda Algorithm updates.

SEO Experts’ Suggestions for Google Panda Algorithm

Suggestions from Danny Sullivan on Google’s Panda Algorithm updates:

  • Google does make small algorithm updates from time to time to lower websites’ rankings in Google’s SERP, but may be unrelated to the Panda algorithm.
  • Google may update a site’s PageRank from time to time that may have negative effect to a site’s ranking in Google’s SERP, but may be unrelated to the Panda algorithm.
  • Google confirmed it may be possible for some websites to be negatively affected by Panda updates to improve their SERP rankings, if the sites have made significant improvements to the quality of content.
  • Monitor your site’s Google organic traffic fluctuation, instead of focusing only on your web pages’ SERP ranking.

Six Revisions’ suggestions on creating websites that are optimized for Google’s Panda Algorithm:

  • Design for Engagement and User Experience
  • Spelling and Grammar are Important
  • Focus on Content Quality
  • Avoid Too Many Ads
  • Avoid Duplicate Content
  • Less is More
  • Ensure High-Quality Code

Posted on May 14, 2012
Category: Google SEO | Leave a Comment

Web Analytics Essentials

I talked about Web Analytics Essentials at Search Engine Strategies (SES) Shanghai 2012 through the recommendation from Eddie Choi (of SES Advisory Board) and Adaline Lau (of Clickz Asia’s Chief Editor):

All slides by gordonchoi on Slideshare.net

Web Analytics Principles and Scope

Google Analytics Basic Setup

  • How does Google Analytics work?
  • Google Analytics account and profile setup
  • Google Analytics Tracking Code (GATC) setup: Standard, multiple sub-domains, multiple domains, Chinese search engines
  • Google Analytics profile filter setup: Lowercase URLs, full path URLs

Web Analytics Reports & KPI Metrics for Web Analysts focusing on SEO and SEM

  • What is a web analyst?
  • Basic reports: Site Traffic (Visit) Trends, Direct vs. Search Engines vs. Referring Sites, New Visitors vs. Returning Visitors, Visitor Frequency (counts of visits) and Visitor Recency (days since last visit), Geography, Browsers, Languages, Mobile Devices, Top Landing Pages vs. Top Pages, SEO/SEM Visit Trends, Top SEO/SEM Keywords, Site Search, In-page Analytics
  • Metrics: Visits, Visitors, Page Views, Unique Page Views, New Visitors, Returning Visitors, Bounce Rate, Exit Rate, Time on Site, Time on Page, Clicks
  • Goals: URL destination, visit duration, page / visit, event
  • Goal funnels
  • Segmentation: Advanced segments
  • Direct Traffic
  • SEM Tracking
  • SEO reports: Traffic trends, spider behavior (pages indexed), page errors, keyword ranking
  • Google Webmaster Tools

Google Analytics Tips

Google Analytics Advanced Setup

  • Social media tracking: Facebook Like/Share buttons, Twitter Tweet button, Google +1 button
  • URL Tracking

Posted on May 2, 2012
Category: Web Analytics | 2 Comments

Social Media Tracking, How Social Affects SEO

I talked about some social media tracking tools at Search Engine Strategies (SES) Shanghai 2012 during the “How Social Affects SEO” panel with two other speakers, Bill Hunt (of Back Azimuth) and TR Harrington, with the help of panel moderator Mike Grehan.

All slides by gordonchoi on Slideshare.net

The social media tracking tools:

Social influence metrics:

  • True reach
  • Amplification
  • Network impact

Posted on April 27, 2012
Category: Web Analytics | Leave a Comment

Google Adwords Conversions 1-per-click, Conversions Many-per-click, Conversion Rate Larger Than 100%

If you have set up and enabled Adwords conversion tracking for your Google Adwords campaigns, you may occasionally see keywords with conversion rates that are larger than 100%.

Why Adwords Conversion Rates Can Be Larger Than 100%?

It is possible to have Adwords conversion rate that is larger than 100% and Google official explains how it can happen:

  • A user clicked one of your Adwords ads, arrived at your site, completed a purchase on your site and reached the conversion confirmation page (or thank you page) of your site (the web page where you have Adwords conversion tracking code inserted). The user left your site and then came back directly to your site without clicking any of your Adwords ad again. This time, the user completed a second purchase and reached the conversion confirmation page again. The result is 2 Adwords conversions are counted, with 1 ad click.
  • A user clicked one of your Adwords ads, arrived at your site, completed a purchase on your site and reached the conversion confirmation page (or thank you page) of your site (the web page where you have Adwords conversion tracking code on). The user clicked away from the conversion confirmation page but then clicked on the browser’s back button to return to the conversion confirmation page (where Adwords conversion tracking code is placed). The result is 2 Adwords conversions are counted, with 1 ad click.

Conversions 1-per-click and Conversions Many-per-click

Conversions 1-per-click and conversions many-per-click are two of the metrics in Google Adwords conversions.

  • Conversions 1-per-click: A conversion is counted for each Adwords ad click leading to a conversion within 30 days of the first ad click. Subsequent conversions that are the result of this first ad click will not be counted as conversion 1-per-click, as conversions 1-per-click will count at most one conversion per ad click. Conversions 1-per-click is most useful when measuring conversions approximating the number of unique customer acquisitions.
  • Conversions Many-per-click: A conversion is counted for each Adwords ad click leading to a conversion within 30 days of the first ad click, and conversions many-per-click will count multiple conversions per click. Conversions Many-per-click is most useful when measuring conversions that happen every time which is actually B2C website transactions.

Metrics related to conversion 1-per-click:

  • Conversion Rate 1-per-click = Conversions 1-per-click / clicks
  • Cost-per-conversion 1-per-click = Cost / conversions 1-per-click

Metrics related to conversion many-per-click:

  • Conversion Rate Many-per-click = Conversions Many-per-click / clicks
  • Cost-per-conversion Many-per-click = Cost / conversions Many-per-click

Conversion Rate Many-per-click may be larger than 100% as you may receive more than one conversion per click.

PPC Metrics

PPC metrics explains conversion, conversion rate and other metrics including Google Adwords Quality Score, impression share and clickthrough rate.

Posted on April 23, 2012
Category: Adwords Tips | Leave a Comment

Tech, Internet Books Collection

When in the business of creating and managing websites and when heading down the road of a webmaster, a search marketer, an online marketer, a social media marketer, a web analyst, a web user interface developer, a web engineer and/or a data warehouse administrator, books written by Avinash Kaushik, Bill Hunt, Steve Krug, Brian Clifton, Malcolm Gladwell, Seth Godin, Steven Levitt, Thomas Friedman, Bill Tancer, Steve McConnell and Ralph Kimball would definitely help you:

  • Search Engine Marketing, Inc.
  • Don’t Make Me Think
  • Rocket Surgery Made Easy
  • Advanced Web Metrics with Google Analytics
  • Web Analytics – An hour a day
  • Web Analytics 2.0
  • Landing Page Optimization
  • Always Be Testing
  • Freakonomics
  • Super Freakonomics
  • Small is the New Big
  • Meatball Sundae
  • Linchpin
  • Outliers
  • Blink
  • The Tipping Point
  • The World is Flat
  • The Google Story
  • Online Marketing Heroes
  • Click
  • Code Complete
  • Rapid Development
  • The Data Warehouse Lifecycle Toolkit

How many books can you recognize?

    Tech, Internet Books Collection

While books provide solid background and/or knowledge of a field (i.e. Web Analytics, SEO, landing page optimization, etc) the list of SEM blogs offer similar topics that are updated more often.

Posted on April 22, 2012
Category: B2C Websites | Leave a Comment

Adwords Not Set Keywords in Google Analytics

When advertising with keyword search campaigns through Google Adwords and tracking traffic and/or goal conversions through Google Analytics for your website, you want Adwords PPC keywords showing up in Google Analytics keyword report as one of the traffic sources:

google / cpc

However, Google Adwords clicks can show up in the Google Analytics keyword report as “(not set)” entry, because Google Analytics couldn’t associate a keyword or any search campaign information with the visits (i.e. Google Adwords clicks).

Link Adwords to Google Analytics

Make sure your Adwords account is always linked to your Google Analytics account. If you run search campaigns on multiple Adwords accounts for your website, then link all multiple Adwords accounts to the Google Analytics account.

Auto-tagging Google Adwords Destination URLs

Turn on auto-tagging and test if auto-tagging works on your site (in case some of the web pages on your website have URL redirection enabled):

  • Append ?gclid=test or &gclid=test parameter to the end of one of your Adwords destination URLs.
  • Put the URL with the appended ?gclid=test or &gclid=test parameter into a web browser address bar.
  • If the web page shows up correctly and the appended ?gclid=test or &gclid=test parameter are still in the web browser address bar, then auto-tagging works and keywords will properly show up in Google Analytics keyword report (within being attributed to the “(not set)” entry).

Manually Tag Google Adwords Destination URLs

In some cases when you have auto-tagging enabled, the amount of “(not set)” entry can become significantly large in the Google Analytics keyword report. You should:

  • Disable auto-tagging in your Google Adwords account.
  • Tag all the destination URLs manually or through Google’s URL Builder, with source (e.g. google), medium (e.g. cpc), campaign, keyword/term and ad/content.

Google Adwords Cost Data in Google Analytics Campaigns Report

For Adwords cost data to show up in Google Analytics campaigns report, when linking Adwords account(s) to Google Analytics account, you should select to apply Adwords cost data.

  • If Adwords cost data is applied and auto-tagging is enabled, then both Adwords cost data and visits data will appear in Google Analytics campaigns report.
  • If Adwords cost data is applied and manual-tagging is used, then both Adwords cost data and visits data will appear in Google Analytics campaigns report.
  • If Adwords cost data is not applied and auto-tagging is enabled, then Adwords cost data will not appear and Adwords dimensions (i.e. campaign name, ad group, keyword) will appear as “(not set)” in Google Analytics campaigns report.
  • If Adwords cost data is not applied and manual-tagging is used, then Adwords visits data without cost data will appear in Google Analytics campaigns report.

Posted on April 4, 2012
Category: Google Analytics | 1 Comment

Google Underwater Web Search

Today Google has this Underwater Search which can be found under the www.google.cn domain. In Chinese, this Google Underwater search is called “Shui Xia Sou Suo”.

http://www.google.cn/landing/shuixia/

Start Google Underwater Search

    Google Underwater Search

A search for “Google Analytics” followed by “Barack Obama” and “China” on Underwater Search.

    Google Underwater Search Results

Google’s Underwater Web Search doesn’t actually return any real search results, but it is rather a joke for April’s Fool Day.

Posted on April 1, 2012
Category: Google | Leave a Comment

Web Performance Optimization, Web Page Load Speed Optimization

Speeding up your site helps:

Google has even made site load speed data available through Google Webmaster Tools and Google Analytics.

Yahoo Developer Network has compiled this document on web performance optimization – 35 best practices for speeding up load time of your website and/or web pages.

  • Minimize HTTP Requests
  • Use a Content Delivery Network
  • Add an Expires or a Cache-Control Header
  • Gzip Components
  • Put Stylesheets at the Top
  • Put Scripts at the Bottom
  • Avoid CSS Expressions
  • Make JavaScript and CSS External
  • Reduce DNS Lookups
  • Minify JavaScript and CSS
  • Avoid Redirects
  • Remove Duplicate Scripts
  • Configure ETags
  • Make Ajax Cacheable
  • Flush the Buffer Early
  • Use GET for AJAX Requests
  • Post-load Components
  • Preload Components
  • Reduce the Number of DOM Elements
  • Split Components Across Domains
  • Minimize the Number of iframes
  • No 404s
  • Reduce Cookie Size
  • Use Cookie-free Domains for Components
  • Minimize DOM Access
  • Develop Smart Event Handlers
  • Choose <link> over @import
  • Avoid Filters
  • Optimize Images
  • Optimize CSS Sprites
  • Don’t Scale Images in HTML
  • Make favicon.ico Small and Cacheable
  • Keep Components under 25K
  • Pack Components into a Multipart Document
  • Avoid Empty Image src

Minimize HTTP Requests

80% of the end-user response time is spent on the front-end. Most of this time is tied up in downloading all the components in the page: images, stylesheets, scripts, Flash, etc. Reducing the number of components in turn reduces the number of HTTP requests required to render the page. This is the key to faster pages.

One way to reduce the number of components in the page is to simplify the page’s design. But is there a way to build pages with richer content while also achieving fast response times? Here are some techniques for reducing the number of HTTP requests, while still supporting rich page designs.

Combined files are a way to reduce the number of HTTP requests by combining all scripts into a single script, and similarly combining all CSS into a single stylesheet. Combining files is more challenging when the scripts and stylesheets vary from page to page, but making this part of your release process improves response times.

CSS Sprites are the preferred method for reducing the number of image requests. Combine your background images into a single image and use the CSS background-image and background-position properties to display the desired image segment.

Image maps combine multiple images into a single image. The overall size is about the same, but reducing the number of HTTP requests speeds up the page. Image maps only work if the images are contiguous in the page, such as a navigation bar. Defining the coordinates of image maps can be tedious and error prone. Using image maps for navigation is not accessible too, so it’s not recommended.

Inline images use the data: URL scheme to embed the image data in the actual page. This can increase the size of your HTML document. Combining inline images into your (cached) stylesheets is a way to reduce HTTP requests and avoid increasing the size of your pages. Inline images are not yet supported across all major browsers.

Reducing the number of HTTP requests in your page is the place to start. This is the most important guideline for improving performance for first time visitors. As described in Tenni Theurer’s blog post Browser Cache Usage – Exposed!, 40-60% of daily visitors to your site come in with an empty cache. Making your page fast for these first time visitors is key to a better user experience.

Use a Content Delivery Network

The user’s proximity to your web server has an impact on response times. Deploying your content across multiple, geographically dispersed servers will make your pages load faster from the user’s perspective. But where should you start?

As a first step to implementing geographically dispersed content, don’t attempt to redesign your web application to work in a distributed architecture. Depending on the application, changing the architecture could include daunting tasks such as synchronizing session state and replicating database transactions across server locations. Attempts to reduce the distance between users and your content could be delayed by, or never pass, this application architecture step.

Remember that 80-90% of the end-user response time is spent downloading all the components in the page: images, stylesheets, scripts, Flash, etc. This is the Performance Golden Rule. Rather than starting with the difficult task of redesigning your application architecture, it’s better to first disperse your static content. This not only achieves a bigger reduction in response times, but it’s easier thanks to content delivery networks.

A content delivery network (CDN) is a collection of web servers distributed across multiple locations to deliver content more efficiently to users. The server selected for delivering content to a specific user is typically based on a measure of network proximity. For example, the server with the fewest network hops or the server with the quickest response time is chosen.

Some large Internet companies own their own CDN, but it’s cost-effective to use a CDN service provider, such as Akamai Technologies, EdgeCast, or level3. For start-up companies and private web sites, the cost of a CDN service can be prohibitive, but as your target audience grows larger and becomes more global, a CDN is necessary to achieve fast response times. At Yahoo!, properties that moved static content off their application web servers to a CDN (both 3rd party as mentioned above as well as Yahoo’s own CDN) improved end-user response times by 20% or more. Switching to a CDN is a relatively easy code change that will dramatically improve the speed of your web site.

Add an Expires or a Cache-Control Header

There are two aspects to this rule:

  • For static components: implement “Never expire” policy by setting far future Expires header
  • For dynamic components: use an appropriate Cache-Control header to help the browser with conditional requests

Web page designs are getting richer and richer, which means more scripts, stylesheets, images, and Flash in the page. A first-time visitor to your page may have to make several HTTP requests, but by using the Expires header you make those components cacheable. This avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.

Browsers (and proxies) use a cache to reduce the number and size of HTTP requests, making web pages load faster. A web server uses the Expires header in the HTTP response to tell the client how long a component can be cached. This is a far future Expires header, telling the browser that this response won’t be stale until April 15, 2010.

Expires: Thu, 15 Apr 2010 20:00:00 GMT

If your server is Apache, use the ExpiresDefault directive to set an expiration date relative to the current date. This example of the ExpiresDefault directive sets the Expires date 10 years out from the time of the request.

ExpiresDefault "access plus 10 years"

Keep in mind, if you use a far future Expires header you have to change the component’s filename whenever the component changes. At Yahoo! we often make this step part of the build process: a version number is embedded in the component’s filename, for example, yahoo_2.0.6.js.

Using a far future Expires header affects page views only after a user has already visited your site. It has no effect on the number of HTTP requests when a user visits your site for the first time and the browser’s cache is empty. Therefore the impact of this performance improvement depends on how often users hit your pages with a primed cache. (A “primed cache” already contains all of the components in the page.) We measured this at Yahoo! and found the number of page views with a primed cache is 75-85%. By using a far future Expires header, you increase the number of components that are cached by the browser and re-used on subsequent page views without sending a single byte over the user’s Internet connection.

Gzip Components

The time it takes to transfer an HTTP request and response across the network can be significantly reduced by decisions made by front-end engineers. It’s true that the end-user’s bandwidth speed, Internet service provider, proximity to peering exchange points, etc. are beyond the control of the development team. But there are other variables that affect response times. Compression reduces response times by reducing the size of the HTTP response.

Starting with HTTP/1.1, web clients indicate support for compression with the Accept-Encoding header in the HTTP request.

Accept-Encoding: gzip, deflate

If the web server sees this header in the request, it may compress the response using one of the methods listed by the client. The web server notifies the web client of this via the Content-Encoding header in the response.

Content-Encoding: gzip

Gzip is the most popular and effective compression method at this time. It was developed by the GNU project and standardized by RFC 1952. The only other compression format you’re likely to see is deflate, but it’s less effective and less popular.

Gzipping generally reduces the response size by about 70%. Approximately 90% of today’s Internet traffic travels through browsers that claim to support gzip. If you use Apache, the module configuring gzip depends on your version: Apache 1.3 uses mod_gzip while Apache 2.x uses mod_deflate.

There are known issues with browsers and proxies that may cause a mismatch in what the browser expects and what it receives with regard to compressed content. Fortunately, these edge cases are dwindling as the use of older browsers drops off. The Apache modules help out by adding appropriate Vary response headers automatically.

Servers choose what to gzip based on file type, but are typically too limited in what they decide to compress. Most web sites gzip their HTML documents. It’s also worthwhile to gzip your scripts and stylesheets, but many web sites miss this opportunity. In fact, it’s worthwhile to compress any text response including XML and JSON. Image and PDF files should not be gzipped because they are already compressed. Trying to gzip them not only wastes CPU but can potentially increase file sizes.

Gzipping as many file types as possible is an easy way to reduce page weight and accelerate the user experience.

Put Stylesheets at the Top

While researching performance at Yahoo!, we discovered that moving stylesheets to the document HEAD makes pages appear to be loading faster. This is because putting stylesheets in the HEAD allows the page to render progressively.

Front-end engineers that care about performance want a page to load progressively; that is, we want the browser to display whatever content it has as soon as possible. This is especially important for pages with a lot of content and for users on slower Internet connections. The importance of giving users visual feedback, such as progress indicators, has been well researched and documented. In our case the HTML page is the progress indicator! When the browser loads the page progressively the header, the navigation bar, the logo at the top, etc. all serve as visual feedback for the user who is waiting for the page. This improves the overall user experience.

The problem with putting stylesheets near the bottom of the document is that it prohibits progressive rendering in many browsers, including Internet Explorer. These browsers block rendering to avoid having to redraw elements of the page if their styles change. The user is stuck viewing a blank white page.

The HTML specification clearly states that stylesheets are to be included in the HEAD of the page: “Unlike A, [LINK] may only appear in the HEAD section of a document, although it may appear any number of times.” Neither of the alternatives, the blank white screen or flash of unstyled content, are worth the risk. The optimal solution is to follow the HTML specification and load your stylesheets in the document HEAD.

Put Scripts at the Bottom

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won’t start any other downloads, even on different hostnames.

In some situations it’s not easy to move scripts to the bottom. If, for example, the script uses document.write to insert part of the page’s content, it can’t be moved lower in the page. There might also be scoping issues. In many cases, there are ways to workaround these situations.

An alternative suggestion that often comes up is to use deferred scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering. Unfortunately, Firefox doesn’t support the DEFER attribute. In Internet Explorer, the script may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.

Avoid CSS Expressions

CSS expressions are a powerful (and dangerous) way to set CSS properties dynamically. They were supported in Internet Explorer starting with version 5, but were deprecated starting with IE8. As an example, the background color could be set to alternate every hour using CSS expressions:

background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );

As shown here, the expression method accepts a JavaScript expression. The CSS property is set to the result of evaluating the JavaScript expression. The expression method is ignored by other browsers, so it is useful for setting properties in Internet Explorer needed to create a consistent experience across browsers.

The problem with expressions is that they are evaluated more frequently than most people expect. Not only are they evaluated when the page is rendered and resized, but also when the page is scrolled and even when the user moves the mouse over the page. Adding a counter to the CSS expression allows us to keep track of when and how often a CSS expression is evaluated. Moving the mouse around the page can easily generate more than 10,000 evaluations.

One way to reduce the number of times your CSS expression is evaluated is to use one-time expressions, where the first time the expression is evaluated it sets the style property to an explicit value, which replaces the CSS expression. If the style property must be set dynamically throughout the life of the page, using event handlers instead of CSS expressions is an alternative approach. If you must use CSS expressions, remember that they may be evaluated thousands of times and could affect the performance of your page.

Make JavaScript and CSS External

Many of these performance rules deal with how external components are managed. However, before these considerations arise you should ask a more basic question: Should JavaScript and CSS be contained in external files, or inlined in the page itself?

Using external files in the real world generally produces faster pages because the JavaScript and CSS files are cached by the browser. JavaScript and CSS that are inlined in HTML documents get downloaded every time the HTML document is requested. This reduces the number of HTTP requests that are needed, but increases the size of the HTML document. On the other hand, if the JavaScript and CSS are in external files cached by the browser, the size of the HTML document is reduced without increasing the number of HTTP requests.

The key factor, then, is the frequency with which external JavaScript and CSS components are cached relative to the number of HTML documents requested. This factor, although difficult to quantify, can be gauged using various metrics. If users on your site have multiple page views per session and many of your pages re-use the same scripts and stylesheets, there is a greater potential benefit from cached external files.

Many web sites fall in the middle of these metrics. For these sites, the best solution generally is to deploy the JavaScript and CSS as external files. The only exception where inlining is preferable is with home pages, such as Yahoo!’s front page and My Yahoo!. Home pages that have few (perhaps only one) page view per session may find that inlining JavaScript and CSS results in faster end-user response times.

For front pages that are typically the first of many page views, there are techniques that leverage the reduction of HTTP requests that inlining provides, as well as the caching benefits achieved through using external files. One such technique is to inline JavaScript and CSS in the front page, but dynamically download the external files after the page has finished loading. Subsequent pages would reference the external files that should already be in the browser’s cache.

Reduce DNS Lookups

The Domain Name System (DNS) maps hostnames to IP addresses, just as phonebooks map people’s names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver contacted by the browser returns that server’s IP address. DNS has a cost. It typically takes 20-120 milliseconds for DNS to lookup the IP address for a given hostname. The browser can’t download anything from this hostname until the DNS lookup is completed.

DNS lookups are cached for better performance. This caching can occur on a special caching server, maintained by the user’s ISP or local area network, but there is also caching that occurs on the individual user’s computer. The DNS information remains in the operating system’s DNS cache (the “DNS Client service” on Microsoft Windows). Most browsers have their own caches, separate from the operating system’s cache. As long as the browser keeps a DNS record in its own cache, it doesn’t bother the operating system with a request for the record.

Internet Explorer caches DNS lookups for 30 minutes by default, as specified by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute, controlled by the network.dnsCacheExpiration configuration setting. (Fasterfox changes this to 1 hour.)

When the client’s DNS cache is empty (for both the browser and the operating system), the number of DNS lookups is equal to the number of unique hostnames in the web page. This includes the hostnames used in the page’s URL, images, script files, stylesheets, Flash objects, etc. Reducing the number of unique hostnames reduces the number of DNS lookups.

Reducing the number of unique hostnames has the potential to reduce the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but reducing parallel downloads may increase response times. My guideline is to split these components across at least two but no more than four hostnames. This results in a good compromise between reducing DNS lookups and allowing a high degree of parallel downloads.

Minify JavaScript and CSS

Minification is the practice of removing unnecessary characters from code to reduce its size thereby improving load times. When code is minified all comments are removed, as well as unneeded white space characters (space, newline, and tab). In the case of JavaScript, this improves response time performance because the size of the downloaded file is reduced. Two popular tools for minifying JavaScript code are JSMin and YUI Compressor. The YUI compressor can also minify CSS.

Obfuscation is an alternative optimization that can be applied to source code. It’s more complex than minification and thus more likely to generate bugs as a result of the obfuscation step itself. In a survey of ten top U.S. web sites, minification achieved a 21% size reduction versus 25% for obfuscation. Although obfuscation has a higher size reduction, minifying JavaScript is less risky.

In addition to minifying external scripts and styles, inlined <script> and <style> blocks can and should also be minified. Even if you gzip your scripts and styles, minifying them will still reduce the size by 5% or more. As the use and size of JavaScript and CSS increases, so will the savings gained by minifying your code.

Avoid Redirects

Redirects are accomplished using the 301 and 302 status codes. Here’s an example of the HTTP headers in a 301 response:

HTTP/1.1 301 Moved Permanently
Location: http://example.com/newuri
Content-Type: text/html

The browser automatically takes the user to the URL specified in the Location field. All the information necessary for a redirect is in the headers. The body of the response is typically empty. Despite their names, neither a 301 nor a 302 response is cached in practice unless additional headers, such as Expires or Cache-Control, indicate it should be. The meta refresh tag and JavaScript are other ways to direct users to a different URL, but if you must do a redirect, the preferred technique is to use the standard 3xx HTTP status codes, primarily to ensure the back button works correctly.

The main thing to remember is that redirects slow down the user experience. Inserting a redirect between the user and the HTML document delays everything in the page since nothing in the page can be rendered and no components can start being downloaded until the HTML document has arrived.

One of the most wasteful redirects happens frequently and web developers are generally not aware of it. It occurs when a trailing slash (/) is missing from a URL that should otherwise have one. For example, going to http://astrology.yahoo.com/astrology results in a 301 response containing a redirect to http://astrology.yahoo.com/astrology/ (notice the added trailing slash). This is fixed in Apache by using Alias or mod_rewrite, or the DirectorySlash directive if you’re using Apache handlers.

Connecting an old web site to a new one is another common use for redirects. Others include connecting different parts of a website and directing the user based on certain conditions (type of browser, type of user account, etc.). Using a redirect to connect two web sites is simple and requires little additional coding. Although using redirects in these situations reduces the complexity for developers, it degrades the user experience. Alternatives for this use of redirects include using Alias and mod_rewrite if the two code paths are hosted on the same server. If a domain name change is the cause of using redirects, an alternative is to create a CNAME (a DNS record that creates an alias pointing from one domain name to another) in combination with Alias or mod_rewrite.

Remove Duplicate Scripts

It hurts performance to include the same JavaScript file twice in one page. This isn’t as unusual as you might think. A review of the ten top U.S. web sites shows that two of them contain a duplicated script. Two main factors increase the odds of a script being duplicated in a single web page: team size and number of scripts. When it does happen, duplicate scripts hurt performance by creating unnecessary HTTP requests and wasted JavaScript execution.

Unnecessary HTTP requests happen in Internet Explorer, but not in Firefox. In Internet Explorer, if an external script is included twice and is not cacheable, it generates two HTTP requests during page loading. Even if the script is cacheable, extra HTTP requests occur when the user reloads the page.

In addition to generating wasteful HTTP requests, time is wasted evaluating the script multiple times. This redundant JavaScript execution happens in both Firefox and Internet Explorer, regardless of whether the script is cacheable.

One way to avoid accidentally including the same script twice is to implement a script management module in your templating system. The typical way to include a script is to use the SCRIPT tag in your HTML page.

<script type="text/javascript" src="menu_1.0.17.js"></script>

An alternative in PHP would be to create a function called insertScript.

<?php insertScript("menu.js") ?>

In addition to preventing the same script from being inserted multiple times, this function could handle other issues with scripts, such as dependency checking and adding version numbers to script filenames to support far future Expires headers.

Configure ETags

Entity tags (ETags) are a mechanism that web servers and browsers use to determine whether the component in the browser’s cache matches the one on the origin server. (An “entity” is another word a “component”: images, scripts, stylesheets, etc.) ETags were added to provide a mechanism for validating entities that is more flexible than the last-modified date. An ETag is a string that uniquely identifies a specific version of a component. The only format constraints are that the string be quoted. The origin server specifies the component’s ETag using the ETag response header.

HTTP/1.1 200 OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag: "10c24bc-4ab-457e1c1f"
Content-Length: 12195

Later, if the browser has to validate a component, it uses the If-None-Match header to pass the ETag back to the origin server. If the ETags match, a 304 status code is returned reducing the response by 12195 bytes for this example.

GET /i/yahoo.gif HTTP/1.1
Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f"
HTTP/1.1 304 Not Modified

The problem with ETags is that they typically are constructed using attributes that make them unique to a specific server hosting a site. ETags won’t match when a browser gets the original component from one server and later tries to validate that component on a different server, a situation that is all too common on Web sites that use a cluster of servers to handle requests. By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.

The ETag format for Apache 1.3 and 2.x is inode-size-timestamp. Although a given file may reside in the same directory across multiple servers, and have the same file size, permissions, timestamp, etc., its inode is different from one server to the next.

IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on IIS is Filetimestamp:ChangeNumber. A ChangeNumber is a counter used to track configuration changes to IIS. It’s unlikely that the ChangeNumber is the same across all IIS servers behind a web site.

The end result is ETags generated by Apache and IIS for the exact same component won’t match from one server to another. If the ETags don’t match, the user doesn’t receive the small, fast 304 response that ETags were designed for; instead, they’ll get a normal 200 response along with all the data for the component. If you host your web site on just one server, this isn’t a problem. But if you have multiple servers hosting your web site, and you’re using Apache or IIS with the default ETag configuration, your users are getting slower pages, your servers have a higher load, you’re consuming greater bandwidth, and proxies aren’t caching your content efficiently. Even if your components have a far future Expires header, a conditional GET request is still made whenever the user hits Reload or Refresh.

If you’re not taking advantage of the flexible validation model that ETags provide, it’s better to just remove the ETag altogether. The Last-Modified header validates based on the component’s timestamp. And removing the ETag reduces the size of the HTTP headers in both the response and subsequent requests. This Microsoft Support article describes how to remove ETags. In Apache, this is done by simply adding the following line to your Apache configuration file:

FileETag none

Make Ajax Cacheable

One of the cited benefits of Ajax is that it provides instantaneous feedback to the user because it requests information asynchronously from the backend web server. However, using Ajax is no guarantee that the user won’t be twiddling his thumbs waiting for those asynchronous JavaScript and XML responses to return. In many applications, whether or not the user is kept waiting depends on how Ajax is used. For example, in a web-based email client the user will be kept waiting for the results of an Ajax request to find all the email messages that match their search criteria. It’s important to remember that “asynchronous” does not imply “instantaneous”.

To improve performance, it’s important to optimize these Ajax responses. The most important way to improve the performance of Ajax is to make the responses cacheable, as discussed in Add an Expires or a Cache-Control Header. Some of the other rules also apply to Ajax:

  • Gzip Components
  • Reduce DNS Lookups
  • Minify JavaScript
  • Avoid Redirects
  • Configure ETags

Let’s look at an example. A Web 2.0 email client might use Ajax to download the user’s address book for autocompletion. If the user hasn’t modified her address book since the last time she used the email web app, the previous address book response could be read from cache if that Ajax response was made cacheable with a future Expires or Cache-Control header. The browser must be informed when to use a previously cached address book response versus requesting a new one. This could be done by adding a timestamp to the address book Ajax URL indicating the last time the user modified her address book, for example, &t=1190241612. If the address book hasn’t been modified since the last download, the timestamp will be the same and the address book will be read from the browser’s cache eliminating an extra HTTP roundtrip. If the user has modified her address book, the timestamp ensures the new URL doesn’t match the cached response, and the browser will request the updated address book entries.

Even though your Ajax responses are created dynamically, and might only be applicable to a single user, they can still be cached. Doing so will make your Web 2.0 apps faster.

Flush the Buffer Early

When users request a page, it can take anywhere from 200 to 500ms for the backend server to stitch together the HTML page. During this time, the browser is idle as it waits for the data to arrive. In PHP you have the function flush(). It allows you to send your partially ready HTML response to the browser so that the browser can start fetching components while your backend is busy with the rest of the HTML page. The benefit is mainly seen on busy backends or light frontends.

A good place to consider flushing is right after the HEAD because the HTML for the head is usually easier to produce and it allows you to include any CSS and JavaScript files for the browser to start fetching in parallel while the backend is still processing. Example:

... <!-- css, js -->
</head>
<?php flush(); ?>
<body>
... <!-- content -->

Yahoo! search pioneered research and real user testing to prove the benefits of using this technique.

Use GET for AJAX Requests

The Yahoo! Mail team found that when using XMLHttpRequest, POST is implemented in the browsers as a two-step process: sending the headers first, then sending data. So it’s best to use GET, which only takes one TCP packet to send (unless you have a lot of cookies). The maximum URL length in IE is 2K, so if you send more than 2K data you might not be able to use GET.

An interesting side affect is that POST without actually posting any data behaves like GET. Based on the HTTP specs, GET is meant for retrieving information, so it makes sense (semantically) to use GET when you’re only requesting data, as opposed to sending data to be stored server-side.

Post-load Components

You can take a closer look at your page and ask yourself: “What’s absolutely required in order to render the page initially?”. The rest of the content and components can wait.

JavaScript is an ideal candidate for splitting before and after the onload event. For example if you have JavaScript code and libraries that do drag and drop and animations, those can wait, because dragging elements on the page comes after the initial rendering. Other places to look for candidates for post-loading include hidden content (content that appears after a user action) and images below the fold.

Tools to help you out in your effort: YUI Image Loader allows you to delay images below the fold and the YUI Get utility is an easy way to include JS and CSS on the fly. For an example in the wild take a look at Yahoo! Home Page with Firebug’s Net Panel turned on.

It’s good when the performance goals are inline with other web development best practices. In this case, the idea of progressive enhancement tells us that JavaScript, when supported, can improve the user experience but you have to make sure the page works even without JavaScript. So after you’ve made sure the page works fine, you can enhance it with some post-loaded scripts that give you more bells and whistles such as drag and drop and animations.

Preload Components

Preload may look like the opposite of post-load, but it actually has a different goal. By preloading components you can take advantage of the time the browser is idle and request components (like images, styles and scripts) you’ll need in the future. This way when the user visits the next page, you could have most of the components already in the cache and your page will load much faster for the user.

There are actually several types of preloading:

  • Unconditional preload – as soon as onload fires, you go ahead and fetch some extra components. Check google.com for an example of how a sprite image is requested onload. This sprite image is not needed on the google.com homepage, but it is needed on the consecutive search result page.
  • Conditional preload – based on a user action you make an educated guess where the user is headed next and preload accordingly. On search.yahoo.com you can see how some extra components are requested after you start typing in the input box.
  • Anticipated preload – preload in advance before launching a redesign. It often happens after a redesign that you hear: “The new site is cool, but it’s slower than before”. Part of the problem could be that the users were visiting your old site with a full cache, but the new one is always an empty cache experience. You can mitigate this side effect by preloading some components before you even launched the redesign. Your old site can use the time the browser is idle and request images and scripts that will be used by the new site.

Reduce the Number of DOM Elements

A complex page means more bytes to download and it also means slower DOM access in JavaScript. It makes a difference if you loop through 500 or 5000 DOM elements on the page when you want to add an event handler for example.

A high number of DOM elements can be a symptom that there’s something that should be improved with the markup of the page without necessarily removing content. Are you using nested tables for layout purposes? Are you throwing in more <div>s only to fix layout issues? Maybe there’s a better and more semantically correct way to do your markup.

A great help with layouts are the YUI CSS utilities: grids.css can help you with the overall layout, fonts.css and reset.css can help you strip away the browser’s defaults formatting. This is a chance to start fresh and think about your markup, for example use <div>s only when it makes sense semantically, and not because it renders a new line.

The number of DOM elements is easy to test, just type in Firebug’s console:

document.getElementsByTagName('*').length

And how many DOM elements are too many? Check other similar pages that have good markup. For example the Yahoo! Home Page is a pretty busy page and still under 700 elements (HTML tags).

Split Components Across Domains

Splitting components allows you to maximize parallel downloads. Make sure you’re using not more than 2-4 domains because of the DNS lookup penalty. For example, you can host your HTML and dynamic content on www.example.org and split static components between static1.example.org and static2.example.org

For more information check “Maximizing Parallel Downloads in the Carpool Lane” by Tenni Theurer and Patty Chi.

Minimize the Number of iframes

Iframes allow an HTML document to be inserted in the parent document. It’s important to understand how iframes work so they can be used effectively.

iframe pros:

  • Helps with slow third-party content like badges and ads
  • Security sandbox
  • Download scripts in parallel

iframe cons:

  • Costly even if blank
  • Blocks page onload
  • Non-semantic

No 404s

HTTP requests are expensive so making an HTTP request and getting a useless response (i.e. 404 Not Found) is totally unnecessary and will slow down the user experience without any benefit.

Some sites have helpful 404s “Did you mean X?”, which is great for the user experience but also wastes server resources (like database, etc). Particularly bad is when the link to an external JavaScript is wrong and the result is a 404. First, this download will block parallel downloads. Next the browser may try to parse the 404 response body as if it were JavaScript code, trying to find something usable in it.

Reduce Cookie Size

HTTP cookies are used for a variety of reasons such as authentication and personalization. Information about cookies is exchanged in the HTTP headers between web servers and browsers. It’s important to keep the size of cookies as low as possible to minimize the impact on the user’s response time.

For more information check “When the Cookie Crumbles” by Tenni Theurer and Patty Chi. The take-home of this research:

  • Eliminate unnecessary cookies
  • Keep cookie sizes as low as possible to minimize the impact on the user response time
  • Be mindful of setting cookies at the appropriate domain level so other sub-domains are not affected
  • Set an Expires date appropriately. An earlier Expires date or none removes the cookie sooner, improving the user response time

Use Cookie-free Domains for Components

When the browser makes a request for a static image and sends cookies together with the request, the server doesn’t have any use for those cookies. So they only create network traffic for no good reason. You should make sure static components are requested with cookie-free requests. Create a subdomain and host all your static components there.

If your domain is www.example.org, you can host your static components on static.example.org. However, if you’ve already set cookies on the top-level domain example.org as opposed to www.example.org, then all the requests to static.example.org will include those cookies. In this case, you can buy a whole new domain, host your static components there, and keep this domain cookie-free. Yahoo! uses yimg.com, YouTube uses ytimg.com, Amazon uses images-amazon.com and so on.

Another benefit of hosting static components on a cookie-free domain is that some proxies might refuse to cache the components that are requested with cookies. On a related note, if you wonder if you should use example.org or www.example.org for your home page, consider the cookie impact. Omitting www leaves you no choice but to write cookies to *.example.org, so for performance reasons it’s best to use the www subdomain and write the cookies to that subdomain.

Minimize DOM Access

Accessing DOM elements with JavaScript is slow so in order to have a more responsive page, you should:
Cache references to accessed elements

  • Update nodes “offline” and then add them to the tree
  • Avoid fixing layout with JavaScript
  • For more information check the YUI theatre’s “High Performance Ajax Applications” by Julien Lecomte.

Develop Smart Event Handlers

Sometimes pages feel less responsive because of too many event handlers attached to different elements of the DOM tree which are then executed too often. That’s why using event delegation is a good approach. If you have 10 buttons inside a div, attach only one event handler to the div wrapper, instead of one handler for each button. Events bubble up so you’ll be able to catch the event and figure out which button it originated from.

You also don’t need to wait for the onload event in order to start doing something with the DOM tree. Often all you need is the element you want to access to be available in the tree. You don’t have to wait for all images to be downloaded. DOMContentLoaded is the event you might consider using instead of onload, but until it’s available in all browsers, you can use the YUI Event utility, which has an onAvailable method.

For more information check the YUI theatre’s “High Performance Ajax Applications” by Julien Lecomte.

Choose <link> over @import

One of the previous best practices states that CSS should be at the top in order to allow for progressive rendering.

In IE @import behaves the same as using <link> at the bottom of the page, so it’s best not to use it.

Avoid Filters

The IE-proprietary AlphaImageLoader filter aims to fix a problem with semi-transparent true color PNGs in IE versions below 7. The problem with this filter is that it blocks rendering and freezes the browser while the image is being downloaded. It also increases memory consumption and is applied per element, not per image, so the problem is multiplied.

The best approach is to avoid AlphaImageLoader completely and use gracefully degrading PNG8 instead, which are fine in IE. If you absolutely need AlphaImageLoader, use the underscore hack _filter as to not penalize your IE7+ users.

Optimize Images

After a designer is done with creating the images for your web page, there are still some things you can try before you FTP those images to your web server.

You can check the GIFs and see if they are using a palette size corresponding to the number of colors in the image. Using imagemagick it’s easy to check using

identify -verbose image.gif

When you see an image useing 4 colors and a 256 color “slots” in the palette, there is room for improvement.

Try converting GIFs to PNGs and see if there is a saving. More often than not, there is. Developers often hesitate to use PNGs due to the limited support in browsers, but this is now a thing of the past. The only real problem is alpha-transparency in true color PNGs, but then again, GIFs are not true color and don’t support variable transparency either. So anything a GIF can do, a palette PNG (PNG8) can do too (except for animations). This simple imagemagick command results in totally safe-to-use PNGs:

convert image.gif image.png
"All we are saying is: Give PiNG a Chance!"

Run pngcrush (or any other PNG optimizer tool) on all your PNGs. Example:

pngcrush image.png -rem alla -reduce -brute result.png

Run jpegtran on all your JPEGs. This tool does lossless JPEG operations such as rotation and can also be used to optimize and remove comments and other useless information (such as EXIF information) from your images.

jpegtran -copy none -optimize -perfect src.jpg dest.jpg

Optimize CSS Sprites

Arranging the images in the sprite horizontally as opposed to vertically usually results in a smaller file size.

Combining similar colors in a sprite helps you keep the color count low, ideally under 256 colors so to fit in a PNG8.

“Be mobile-friendly” and don’t leave big gaps between the images in a sprite. This doesn’t affect the file size as much but requires less memory for the user agent to decompress the image into a pixel map. 100×100 image is 10 thousand pixels, where 1000×1000 is 1 million pixels

Don’t Scale Images in HTML

Don’t use a bigger image than you need just because you can set the width and height in HTML. If you need

<img width="100" height="100" src="mycat.jpg" alt="My Cat" />

then your image (mycat.jpg) should be 100x100px rather than a scaled down 500x500px image.

Make favicon.ico Small and Cacheable

The favicon.ico is an image that stays in the root of your server. It’s a necessary evil because even if you don’t care about it the browser will still request it, so it’s better not to respond with a 404 Not Found. Also since it’s on the same server, cookies are sent every time it’s requested. This image also interferes with the download sequence, for example in IE when you request extra components in the onload, the favicon will be downloaded before these extra components.

So to mitigate the drawbacks of having a favicon.ico make sure:

  • It’s small, preferably under 1K.
  • Set Expires header with what you feel comfortable (since you cannot rename it if you decide to change it). You can probably safely set the Expires header a few months in the future. You can check the last modified date of your current favicon.ico to make an informed decision.

Imagemagick can help you create small favicons.

Keep Components under 25K

This restriction is related to the fact that iPhone won’t cache components bigger than 25K. Note that this is the uncompressed size. This is where minification is important because gzip alone may not be sufficient.

For more information check “Performance Research, Part 5: iPhone Cacheability – Making it Stick” by Wayne Shea and Tenni Theurer.

Pack Components into a Multipart Document

Packing components into a multipart document is like an email with attachments, it helps you fetch several components with one HTTP request (remember: HTTP requests are expensive). When you use this technique, first check if the user agent supports it (iPhone does not).

Avoid Empty Image src

Image with empty string src attribute occurs more than one will expect. It appears in two form:

1. straight HTML
<img src="">

2. JavaScript
var img = new Image();
img.src = "";

Both forms cause the same effect: browser makes another request to your server.

  • Internet Explorer makes a request to the directory in which the page is located.
  • Safari and Chrome make a request to the actual page itself.
  • Firefox 3 and earlier versions behave the same as Safari and Chrome, but version 3.5 addressed this issue[bug 444931] and no longer sends a request.
  • Opera does not do anything when an empty image src is encountered.

Why is this behavior bad?

  • Cripple your servers by sending a large amount of unexpected traffic, especially for pages that get millions of page views per day.
  • Waste server computing cycles generating a page that will never be viewed.
  • Possibly corrupt user data. If you are tracking state in the request, either by cookies or in another way, you have the possibility of destroying data. Even though the image request does not return an image, all of the headers are read and accepted by the browser, including all cookies. While the rest of the response is thrown away, the damage may already be done.

The root cause of this behavior is the way that URI resolution is performed in browsers. This behavior is defined in RFC 3986 – Uniform Resource Identifiers. When an empty string is encountered as a URI, it is considered a relative URI and is resolved according to the algorithm defined in section 5.2. This specific example, an empty string, is listed in section 5.4. Firefox, Safari, and Chrome are all resolving an empty string correctly per the specification, while Internet Explorer is resolving it incorrectly, apparently in line with an earlier version of the specification, RFC 2396 – Uniform Resource Identifiers (this was obsoleted by RFC 3986). So technically, the browsers are doing what they are supposed to do to resolve relative URIs. The problem is that in this context, the empty string is clearly unintentional.

HTML5 adds to the description of the tag’s src attribute to instruct browsers not to make an additional request in section 4.8.2:

  • The src attribute must be present, and must contain a valid URL referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted. If the base URI of the element is the same as the document’s address, then the src attribute’s value must not be the empty string.

Hopefully, browsers will not have this problem in the future. Unfortunately, there is no such clause for <script src=”"> and <link href=”">. Maybe there is still time to make that adjustment to ensure browsers don’t accidentally implement this behavior.

This rule was inspired by Yahoo!’s JavaScript guru Nicolas C. Zakas. For more information check out his article “Empty image src can destroy your site“.

Posted on March 26, 2012
Category: Landing Page Optimization | 1 Comment

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