Creating high-performance web sites or applications is essential for web developers.
Jatinder Mann explains that the goal of a developer is to improve web performance by reducing 25 factors:
Display time
The most important objective is what we refer to as 'Display Time'. This has many names across the industry including 'time to glass' and 'primary paint'. Display Time measures the time from when the user performs an action until the user sees the result of that action on the screen. This is the duration from when the user navigates to the site until when the site is visually complete loading.
Elapsed time
Most sites continue to perform work in response to the user action after the content has been displayed to the screen. This may include downloading user data (such as email messaging) or sending analytics back to a provider. From users' perspective, the site may appear loaded. However, significant work is often occurring in the background, which impacts responsiveness.
CPU time
Web browsers are almost exclusively limited by the CPU; the work a browser performs on the CPU and how efficiently that work occurs will make the single largest impact on performance. That's why offloading work to the GPU has made such a significant impact to IE9 and IE10 performance. The amount of CPU time required to perform the action and the CPU efficiency are critical.
Resource utilisation
Building a fast browser means ensuring resources across the entire PC work well together. This includes network utilisation, memory usage patterns, GPU processing, graphics, memory and hundreds of other dimensions. Since customers run several applications at the same time on their PC, it's important for browsers to responsibly share these resources with other applications.
Power consumption
When utilising underlying PC hardware, it's important to take power consumption into consideration. The more efficiently a browser uses power, the longer batteries will last in mobile scenarios, the lower the electricity costs for operating the device, and the smaller the environmental impact. Power and performance are complementary goals.
I want to share with you some of the things that I've learned about developing faster websites and apps, and the changes you can make today to improve performance.
There are seven key principles that developers can take into consideration:
- Respond quickly to network requests
- Minimise bytes to download
- Efficiently structure markup
- Optimise media usage
- Write fast JavaScript
- Render in standards mode
- Monitor what your app is doing
Within these seven principles I've included a number of helpful performance tips below that will make your HTML5 website sand apps run faster (watch a video of a presentation I made for even more HTML5 tips).
Respond quickly to network requests
01. Prevent 3xx redirections
When a user clicks on a link, they expect to receive content as quickly as possible. 3xx redirections can create a 250 millisecond delay in an application. This may seem like a short delay, but it's roughly 10 per cent of page load time. Around 63 per cent of the world's top websites contain 3xx redirections.
02. Use content distribution networks (CDNs)
With CDNs you can easily geographically locate your data closer to your user. Services such as Azure help with this and can help to reduce the amount of time that content spends travelling between locations. On today's networks, that can save up to as much as 300 milliseconds.
03. Maximise concurrent connections
When people think about a network, they often think of a single pipeline conducting content back and forth. In fact, the browser can make six concurrent connections at a given time, enabling it to download six resources at once. This is possible across multiple domains: by distributing your content you can further increase the number of resources you can download simultaneously. If your website holds images across six or seven domains, you can significantly reduce the amount of time the page takes to load.
04. Understand your network timing
Understand the breakdown of your network timing – navigation timing, resource timing and user timing – and use standards-based APIs available in modern browsers such as IE10, and in Windows Store apps. You can get higher resolution timing information on the navigation of your document. Navigation timing allows you to understand the amount of time your application spends in various phases.
Minimise bytes downloaded
05. Download fewer resources and bytes
The fewer resources you can download the better. Look at the resources you are downloading and work out where you can cut down. The average website today downloads 777kB of data. The vast majority of these bytes are taken up by images, followed by JavaScript and Flash content.
06. Gzip: compress network traffic
The best way to download fewer bytes is to Gzip your content. Most people will get this service for free because of the servers they are using, but many unintentionally turn off this decoding technique.
07. Standard file capitalisation
This often catches people by surprise, but the server will pick up on variations in upper/ lower case. The following download requests are two variations of the same request:
Lower case
- <img src="icon.png" />
Title case
- <img src="Icon.png" />
From the web platform perspective, these are two different files. Therefore, two different network requests will be processed as a result.
Efficiently structure markup
08. Avoid quirks mode
Always use a standards-based doctype to avoid quirks mode. Start with . The modern web has no place for quirks mode, which was designed so that mid-90s web pages would be usable in turn-of-the-century 'modern' browsers like IE6 and Firefox 2.
Most web pages today end up in quirks mode accidentally, as a consequence of an invalid doctype or extraneous text before the doctype. This can cause strange layout issues that are hard to debug.
09. Avoid inline JavaScript events
Take care that you don't use inline JavaScript events inside your HTML markup. One example of this would be . This is a practice that breaks the clean separation that should exist between the markup, presentation and the behaviour.
Also, if your scripts are loading at the bottom of the file, it's possible for a user to interact with the page, trigger an event and attempt to call a script that hasn't loaded yet, which will cause an error.
10. Link style sheets at the top of the page
By placing the CSS at the top of the page, the browser will issue that request first and simultaneously block painting until CSS is complete. By placing CSS in the head, images, JavaScript and other time intensive resources can be downloaded later. To that end, we would also advise against linking your CSS at the bottom of the page.
11. Only include the necessary styles
It may seem like a good idea to hold one very large CSS file that's shared across the entirety of your website. Some of the top news sites in the world use this approach today. One news site, for example, has one style sheet with 4,000 rules in it, of which only 5-10 per cent are used in a single page. The browser has to download all these styles, pass them and create internal data structures, most of which will never be used.
12. Link your JavaScript at the bottom of the page
This is common best practice. You should always make sure that the styles and the visuals are downloaded first. That way, the JavaScript can follow later and manipulate the page how it likes. However, if you absolutely have to link the JavaScript in the header, as determined by the CRM system or hosting service you are using, then be sure to use the defer tag.
13. Understand the limits of HTML5 tags
New HTML5 tags like