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The Front-End Performance Checklist speeds up your web developments —
An exhaustive list of all the elements that will speed up your current web developments and offer the best user experience.

David Dias

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Performance is a huge subject, but it's not always a "back-end" or an "admin" subject: it's a Front-End responsibility too. The Front-End Performance Checklist is an exhaustive list of elements you should check or at least be aware of, as a Front-End developer and apply to your project (personal and professional).

How to use?

For each rule, you will have a paragraph explaining why this rule is important and how you can fix it. For more deep information, you should find links that will point to 🛠 tools, 📖 articles or 📹 medias that can complete the checklist.

All items in the Front-End Performance Checklist are essentials to achieve the highest performance score but you would find an indicator to help you to eventually prioritised some rules amount others. There are 3 levels of priority:

  • ![Low][low] means that the item has a low priority.
  • ![Medium][medium] means that the item has a medium priority. You shouldn't avoid tackling that item.
  • ![High][high] means that the item has a high priority. You can't avoid following that rule and implement the corrections recommended.

Performance tools

List of the tools you can use to test or monitor your website or application:

References

HTML

  • Minified HTML: ![medium] The HTML code is minified, comments, white spaces and new lines are removed from production files.

    Why:

    Removing all unnecessary spaces, comments and attributes will reduce the size of your HTML and speed up your site's page load times and obviously lighten the download for your user.

    How:

    Most of the frameworks have plugins to facilitate the minification of the webpages. You can use a bunch of NPM modules that can do the job for you automatically.

  • Place CSS tags always before JavaScript tags: ![high] Ensure that your CSS is always loaded before having JavaScript code.

    <!-- Not recommended -->
<script src="jquery.js"></script>
<script src="foo.js"></script>
<link rel="stylesheet" href="foo.css" />

<!-- Recommended -->
<link rel="stylesheet" href="foo.css" />
<script src="jquery.js"></script>
<script src="foo.js"></script>

    Why:

    Having your CSS tags before any JavaScript enables better, parallel download which speed up browser rendering time.

    How:

    ⁃ Ensure that <link> and <style> in your <head> are always before your <script>.

  • Minimize the number of iframes: ![high] Use iframes only if you don't have any other technical possibility. Try to avoid iframes as much as you can.

  • Pre-load optimization with prefetch, dns-prefetch and prerender: ![low] Popular browsers can use directive on <link> tag and "rel" attribute with certain keywords to pre-load specific URLs.

    Why:

    Prefetching allows a browser to silently fetch the necessary resources needed to display content that a user might access in the near future. The browser is able to store these resources in its cache and speed up the way web pages load when they are using different domains for page resources. When a web page has finished loading and the idle time has passed, the browser begins downloading other resources. When a user go in a particular link (already prefetched), the content will be instantly served.

    How:

    ⁃ Ensure that <link> is in your <head> section.

CSS

  • Minification: ![high] All CSS files are minified, comments, white spaces and new lines are removed from production files.

    Why:

    When CSS files are minified, the content is loaded faster and less data is sent to the client. It's important to always minify CSS files in production. It is beneficial for the user as it is for any business who wants to lower bandwidth costs and lower resource usage.

    How:

    ⁃ Use tools to minify your files automatically before or during your build or your deployment.

  • Concatenation: ![medium] CSS files are concatenated in a single file (Not always valid for HTTP/2).

    <!-- Not recommended -->
<link rel="stylesheet" href="foo.css" />
<link rel="stylesheet" href="bar.css" />

<!-- Recommended -->
<link rel="stylesheet" href="foobar.css" />

    Why:

    If you are still using HTTP/1, you may need to still concatenate your files, it's less true if your server use HTTP/2 (tests should be made).

    How:

    ⁃ Use online tool or any plugin before or during your build or your deployment to concatenate your files. ⁃ Ensure, of course, that concatenation does not break your project.

  • Non-blocking: ![high] CSS files need to be non-blocking to prevent the DOM from taking time to load.

    <link rel="preload" href="global.min.css" as="style" onload="this.rel='stylesheet'" />
<noscript><link rel="stylesheet" href="global.min.css" /></noscript>

    Why:

    CSS files can block the page load and delay the rendering of your page. Using preload can actually load the CSS files before the browser starts showing the content of the page.

    How:

    ⁃ You need to add the rel attribute with the preload value and add as="style" on the <link> element.

  • Unused CSS: ![medium] Remove unused CSS selectors.

    Why:

    Removing unused CSS selectors can reduce the size of your files and then speed up the load of your assets.

    How:

    ⁃ ⚠️ Always check if the framework CSS you want to use don't already has a reset / normalize code included. Sometimes you may not need everything that is inside your reset / normalize file.

  • CSS Critical: ![high] The CSS critical (or "above the fold") collects all the CSS used to render the visible portion of the page. It is embedded before your principal CSS call and between <style></style> in a single line (minified if possible).

    Why:

    Inlining critical CSS help to speed up the rendering of the web pages reducing the number of requests to the server.

    How:

    Generate the CSS critical with online tools or using a plugin like the one that Addy Osmani developed.

  • Embedded or inline CSS: ![high] Avoid using embed or inline CSS inside your <body> (Not valid for HTTP/2)

    Why:

    One of the first reason it's because it's a good practice to separate content from design. It also helps you have a more maintainable code and keep your site accessible. But regarding performance, it's simply because it decreases the file-size of your HTML pages and the load time.

    How:

    Always use external stylesheets or embed CSS in your <head> (and follow the others CSS performance rules)

  • Analyse stylesheets complexity: ![high] Analyzing your stylesheets can help you to flag issues, redundancies and duplicate CSS selectors.

    Why:

    Sometimes you may have redundancies or validation errors in your CSS, analysing your CSS files and removed these complexities can help you to speed up your CSS files (because your browser will read them faster)

    How:

    Your CSS should be organized, using a CSS preprocessor can help you with that. Some online tools listed below can also help you analysing and correct your code.

Fonts

![fonts]

Images

JavaScript

Server

Performances and JS Frameworks

Angular

React

Vue

Performances and CMS

WordPress

Articles

Plugins recommended

Translations

The Front-End Performance Checklist wants to also be available in other languages! Don't hesitate to submit your contribution!

Originally published on Codeburst

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