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The Front-End Performance Checklist speeds up your web developments

25 min read

This article was originally published on Codeburst

Introduction

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 ๐Ÿ›  tools, ๐Ÿ“– articles or ๐Ÿ“น 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

![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:

    โƒ 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:

    โƒ Ensure that <link> is in your <head> section.

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CSS

![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:

    โƒ 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:

    โƒ Use online tool or any plugin before or during your build or your deployment to concatenate your files. โƒ 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:

    โƒ 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:

    โƒ โš ๏ธ 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.
    
    * ๐Ÿ“– [Understanding Critical CSS](https://www.smashingmagazine.com/2015/08/understanding-critical-css/)
    * ๐Ÿ›  [Critical by Addy Osmani on GitHub](https://github.com/addyosmani/critical) automates this.
    * ๐Ÿ“– [Inlining critical CSS for better web performance | Go Make Things](https://gomakethings.com/inlining-critical-css-for-better-web-performance/)
     * ๐Ÿ›  [Critical Path CSS Generator - Prioritize above the fold content :: SiteLocity](https://www.sitelocity.com/critical-path-css-generator)
     * ๐Ÿ“– [Reduce the size of the above-the-fold content
    

    ](https://developers.google.com/speed/docs/insights/PrioritizeVisibleContent)

  • 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.

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Fonts

![fonts]

  • Prevent Flash or Invisible Text: ![medium] Avoid transparent text until the Webfont is loaded

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Images

![images]

  • Use vector image vs raster/bitmap: ![medium] Prefer using vector image rather than bitmap images (when possible).

    Why:

    Vector images (SVG) tend to be smaller than images and SVG's are responsive and scale perfectly. These images can be animated and modified by CSS.

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JavaScript

![javascript]

  • JS Minification: ![high] All JavaScript files are minified, comments, white spaces and new lines are removed from production files (still valid if using HTTP/2).

    Why:

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

    How:

    โƒ Use the tools suggested below to minify your files automatically before or during your build or your deployment.

  • No JavaScript inside: ![medium] (Only valid for website) Avoid having multiple JavaScript codes embedded in the middle of your body. Regroup your JavaScript code inside external files or eventually in the <head> or at the end of your page (before </body>).

    Why:

    Placing JavaScript embedded code directly in your <body> can slow down your page because it loads while the DOM is being built. The best option is to use external files with async or defer to avoid blocking the DOM. Another option is to place some scripts inside your <head>. Most of the time analytics code or small script that need to load before the DOM gets to main processing.

    How:

    Ensure that all your files are loaded using async or defer and decide wisely the code that you will need to inject in your <head>.

  • Non-blocking JavaScript: ![high] JavaScript files are loaded asynchronously using async or deferred using defer attribute.

    <!-- Defer Attribute -->
    <script defer src="foo.js"></script>
    
    <!-- Async Attribute -->
    <script async src="foo.js"></script>
    

    Why:

    JavaScript blocks the normal parsing of the HTML document, so when the parser reaches a <script> tag (particularly is inside the <head>), it stops to fetch and run it. Adding async or defer are highly recommended if your scripts are placed in the top of your page but less valuable if just before your </body> tag. But it's a good practice to always use these attributes to avoid any performance issue.

    How:

    โƒ Add async (if the script don't rely on other scripts) or defer (if the script relies upon or relied upon by an async script) as an attribute to your script tag. โƒ If you have small scripts, maybe use inline script place above async scripts.

  • Optimized and updated JS libraries: ![medium] All JavaScript libraries used in your project are necessary (prefer Vanilla JavaScript for simple functionalities), updated to their latest version and don't overwhelm your JavaScript with unnecessary methods.

    Why:

    Most of the time, new versions come with optimization and security fix. You should use the most optimized code to speed up your project and ensure that you'll not slow down your website or app without outdated plugin.

    How:

    If your project use NPM packages, npm-check is a pretty interesting library to upgrade / update your libraries. Greenkeeper can automatically look for your dependencies and suggest an update every time a new version is out.

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Server

![server-side]

  • Your website is using HTTPS: ![high]

    Why:

    HTTPS is not only for ecommerce websites, but for all websites that are exchanging data. Data shared by a user or data shared to an external entity. Modern browsers today limit functionalities for sites that are not secure. For example: geolocation, push notifications and service workers don't work if your instance is not using HTTPS. And today is much more easy to setup a project with an SSL certificate than it was before (and for free, thanks to Let's Encrypt).

  • Minimizing HTTP requests: ![high] Always ensure that every file requested are essential for your website or application.
  • Use a CDN to deliver your assets: ![medium] Use a CDN to deliver faster your content over the world.
  • Serve files from the same protocol: ![high] Avoid having your website serving files coming from source using HTTP on your website which is using HTTPS for example. If your website is using HTTPS, external files should come from the same protocol.

  • Serve reachable files: ![high] Avoid requesting unreachable files (404).

  • Set HTTP cache headers properly: ![high] Set HTTP headers to avoid expensive number of roundtrips between your browser and the server.
  • GZIP / Brotli compression is enabled: ![high] Use a compression method such as Gzip or Brotli to reduce the size of your JavaScript files. With a smaller sizes file, users will be able to download the asset faster, resulting in improved performance.

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Performances and JS Frameworks

Angular

React

Vue

Performances and CMS

WordPress

Articles


Translations

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

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About the author

David Dias is a Software Engineer and UX/UI passionate. He tries to document everything he learns and help people starting their coding careers. After living in France, Brazil and Mauritius, he now lives in Toronto, Canada where he works for HomeX.

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