🎮 The only Front-End Performance Checklist that runs faster than the others
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Front-End Performance Checklist
One simple rule: "Design and code with performance in mind"
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).
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:
List of the tools you can use to test or monitor your website or application:
 Minified HTML: The HTML code is minified, comments, white spaces and new lines are removed from production files.
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.
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.
<!-- 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>
⁃ Ensure that
are always before your
 Pre-load optimization with prefetch, dns-prefetch and prerender: Popular browsers can use directive on
tag and "rel" attribute with certain keywords to pre-load specific URLs.
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.
⁃ Ensure that
is in your
 Minification: All CSS files are minified, comments, white spaces and new lines are removed from production files.
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.
⁃ Use tools to minify your files automatically before or during your build or your deployment.
 Concatenation: 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">
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).
⁃ 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: 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>
CSS files can block the page load and delay the rendering of your page. Using
can actually load the CSS files before the browser starts showing the content of the page.
⁃ You need to add the
attribute with the
value and add
 Unused CSS: Remove unused CSS selectors.
Removing unused CSS selectors can reduce the size of your files and then speed up the load of your assets.
⁃ ⚠️ 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: 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
in a single line (minified if possible).
Inlining critical CSS help to speed up the rendering of the web pages reducing the number of requests to the server.
Generate the CSS critical with online tools or using a plugin like the one that Addy Osmani developed.
 Embedded or inline CSS: Avoid using embed or inline CSS inside your
(Not valid for HTTP/2)
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.
Always use external stylesheets or embed CSS in your
(and follow the others CSS performance rules)
 Analyse stylesheets complexity: Analyzing your stylesheets can help you to flag issues, redundancies and duplicate CSS selectors.
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)
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.
 Webfont formats: You are using WOFF2 on your web project or application.
According to Google, the WOFF 2.0 Web Font compression format offers 30% average gain over WOFF 1.0. It's then good to use WOFF 2.0, WOFF 1.0 as a fallback and TTF.
Check before buying your new font that the provider gives you the WOFF2 format. If you are using a free font, you can always use Font Squirrel to generate all the formats you need.
to load your fonts faster:**
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
When you arrived on a website, your device needs to find out where your site lives and which server it needs to connect with. Your browser had to contact a DNS server and wait for the lookup complete before fetching the resource (fonts, CSS files...). Prefetches and preconnects allow the browser to lookup the DNS information and start establishing a TCP connection to the server hosting the font file. This provides a performance boost because by the time the browser gets around to parsing the css file with the font information and discovering it needs to request a font file from the server, it will already have pre-resolved the DNS information and have an open connection to the server ready in its connection pool.
⁃ Before prefetching your webfonts, use webpagetest to evaluate your website
⁃ Look for teal colored DNS lookups and note the host that are being requested
⁃ Prefetch your webfonts in your
and add eventually these hostnames that you should prefetch too
 Webfont size: Webfont sizes don't exceed 300kb (all variants included)
 Prevent Flash or Invisible Text: Avoid transparent text until the Webfont is loaded
for the Masses](https://css-tricks.com/font-display-masses/)
Optimized images load faster in your browser and consume less data.
⁃ Try using CSS3 effects when it's possible (instead of a small image)
⁃ When it's possible, use fonts instead of text encoded in your images
⁃ Use SVG
⁃ Use a tool and specify a level compression under 85. * 📖 Image Optimization | Web Fundamentals | Google Developers* 📖 Essential Image Optimization - An eBook by Addy Osmani* 🛠 TinyJPG – Compress JPEG images intelligently* 🛠 Kraken.io - Online Image Optimizer* 🛠 Compressor.io - optimize and compress JPEG photos and PNG images* 🛠 Cloudinary - Image Analysis Tool* 🛠 ImageEngine - Image Webpage Loading Test* 🛠 SVGOMG - Optimize SVG vector graphics files*  Images format: Choose your image format appropriately.
To ensure that your images don't slow your website, choose the format that will correspond to your image. If it's a photo, JPEG is most of the time more appropriate than PNG or GIF. But don't forget to look a the nex-gen formats which can reduce the size of your files. Each image format has pros and cons, it's important to know these to make the best choice possible.
⁃ Use Lighthouse to identify which images can eventually use next-gen formats (like JPEG 2000m JPEG XR or WebP)
⁃ Compare different formats, sometimes using PNG8 is better than PNG16, sometimes it's not. * 📖 Serve Images in Next-Gen Formats | Tools for Web Developers | Google Developers* 📖 What Is the Right Image Format for Your Website? — SitePoint* 📖 PNG8 - The Clear Winner — SitePoint* 📖 8-bit vs 16-bit - What Color Depth You Should Use And Why It Matters - DIY Photography-  Use vector image vs raster/bitmap: Prefer using vector image rather than bitmap images (when possible).
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. *  Images dimensions: Set
if the final rendered image size is known.
If height and width are set, the space required for the image is reserved when the page is loaded. However, without these attributes, the browser does not know the size of the image, and cannot reserve the appropriate space to it. The effect will be that the page layout will change during loading (while the images load). *  Avoid using Base64 images: You could eventually convert tiny images to base64 but it's actually not the best practice. * 📖 Base64 Encoding & Performance, Part 1 and 2 by Harry Roberts* 📖 A closer look at Base64 image performance – The Page Not Found Blog* 📖 When to base64 encode images (and when not to) | David Calhoun* 📖 Base64 encoding images for faster pages | Performance and seo factors*  Lazy loading: Offscreen images are loaded lazily (A noscript fallback is always provided).
It will improve the response time of the current page and then avoid loading unnecessary images that the user may not need.
⁃ Also make sure to lazyload alternative images shown at mouseover or upon other user actions. * 🛠 verlok/lazyload: GitHub* 🛠 aFarkas/lazysizes: GitHub* 🛠 mfranzke/loading-attribute-polyfill: GitHub* 📖 Lazy Loading Images and Video | Web Fundamentals | Google Developers* 📖 5 Brilliant Ways to Lazy Load Images For Faster Page Loads - Dynamic Drive Blog*  Responsive images: Ensure to serve images that are close to your display size.
Small devices don't need images bigger than their viewport. It's recommended to have multiple versions of one image on different sizes.
⁃ Create different image sizes for the devices you want to target.
to deliver multiple variants of each image. * 📖 Responsive images - Learn web development | MDN
⁃ Use the tools suggested below to minify your files automatically before or during your build or your deployment.
or at the end of your page (before
can slow down your page because it loads while the DOM is being built. The best option is to use external files with
to avoid blocking the DOM. Another option is to place some scripts inside your
. Most of the time analytics code or small script that need to load before the DOM gets to main processing.
Ensure that all your files are loaded using
and decide wisely the code that you will need to inject in your
or deferred using
<!-- Defer Attribute --><script defer src="foo.js"></script><!-- Async Attribute --><script async src="foo.js"></script>
tag (particularly is inside the
), it stops to fetch and run it. Adding
are highly recommended if your scripts are placed in the top of your page but less valuable if just before your
tag. But it's a good practice to always use these attributes to avoid any performance issue.
(if the script don't rely on other scripts) or
(if the script relies upon or relied upon by an async script) as an attribute to your script tag.
⁃ If you have small scripts, maybe use inline script place above async scripts.
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.
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.
 Check dependencies size limit: Ensure to use wisely external libraries, most of the time, you can use a lighter library for a same functionality.
You may be tempted to use one of the 745 000 packages you can find on npm, but you need to choose the best package for your needs. For example, MomentJS is an awesome library but with a lot of methods you may never use, that's why Day.js was created. It's just 2kB vs 16.4kB gz for Moment.
Always compare and choose the best and lighter library for your needs. You can also use tools like npm trends to compare NPM package downloads counts or Bundlephobia to know the size of your dependencies.
Use the Timeline tool in the Chrome Developer Tool to evaluate scripts events and found the one that may take too much time.
 Use of Service Workers: You are using Service Workers in your PWA to cache data or execute possible heavy tasks without impacting the user experience of your application.
 Your website is using HTTPS:
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).
 Page weight < 1500 KB (ideally < 500 KB): Reduce the size of your page + resources as much as you can.
Ideally you should try to target < 500 KB but the state of web shows that the median of Kilobytes is around 1500 KB (even on mobile). Depending on your target users, network connection, devices, it's important to reduce as much as possible your total Kilobytes to have the best user experience possible.
⁃ All the rules inside the Front-End Performance Checklist will help you to reduce as much as possible your resources and your code.
 Page load times < 3 seconds: Reduce as much as possible your page load times to quickly deliver your content to your users.
Faster your website or app is, less you have probability of bounce increases, in other terms you have less chances to lose your user or future client. Enough researches on the subject prove that point.
 Time To First Byte < 1.3 seconds: Reduce as much as you can the time your browser waits before receiving data.
 Cookie size: If you are using cookies, be sure each cookie doesn't exceed 4096 bytes and your domain name doesn't have more than 20 cookies.
Cookies are 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.
Eliminate unnecessary cookies.
 Minimizing HTTP requests: Always ensure that every file requested are essential for your website or application.
 Use a CDN to deliver your assets: Use a CDN to deliver faster your content over the world.
 Serve reachable files: Avoid requesting unreachable files (404).
 Set HTTP cache headers properly: Set HTTP headers to avoid expensive number of roundtrips between your browser and the server.
The Front-End Performance Checklist wants to also be available in other languages! Don't hesitate to submit your contribution!
Open an issue or a pull request to suggest changes or additions.
If you have any question or suggestion, don't hesitate to use Discord or Twitter:
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