The complexity only grows as the project becomes larger. While it's impossible to plan for every change that could possibly happen over the life of a project, you can help future-proof y This project-based eBook will introduce you to Cascading Style Sheets CSS , a stylesheet language used to control the presentation of websites, by building a personal website using our demonstration site as a model.
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This behavior was a default in Safari for years. Initiate a high-priority, asynchronous fetch for the CSS file. Works in - most modern browsers. Works in all browsers with JavaScript enabled. In the unlikely event that a visitor has intentionally disabled - JavaScript, fall back to the original method.
The good news is that, - although this is a render-blocking request, it can still make use of the - preconnect which makes it marginally faster than the default. Then we initiate a high-priority, asynchronous fetch for the CSS file. Plus, the support for font-display was added recently to Google Fonts as well, so we can use it out of the box. A quick word of caution though.
If you use font-display: optional , it might be suboptimal to also use preload as it will trigger that web font request early causing network congestion if you have other critical path resources that need to be fetched. Use preconnect for faster cross-origin font requests, but be cautious with preload as preloading fonts from a different origin wlll incur network contention. On the other hand, it might be a good idea to opt out of web fonts or at least second stage render if the user has enabled Reduce Motion in accessibility preferences or has opted in for Data Saver Mode see Save-Data header , or when the user has a slow connectivity via Network Information API.
We can also use the prefers-reduced-data CSS media query to not define font declarations if the user has opted into data-saving mode there are other use-cases, too. Currently supported only in Chrome and Edge behind a flag. To measure the web font loading performance, consider the All Text Visible metric the moment when all fonts have loaded and all content is displayed in web fonts , Time to Real Italics as well as Web Font Reflow Count after first render.
Obviously, the lower both metrics are, the better the performance is. What about variable fonts , you might ask? They give us a much broader design space for typographic choices, but it comes at the cost of a single serial request opposed to a number of individual file requests. While variable fonts drastically reduce the overall combined file size of font files , that single request might be slow, blocking the rendering of all content on a page. So subsetting and splitting the font into character sets still matter.
Now, what would make a bulletproof web font loading strategy then? On the first visit, inject the preloading of scripts just before the blocking external scripts. Alternatively, to emulate a web font for a fallback font, we can use font-face descriptors to override font metrics demo , enabled in Chrome Note that adjustments are complicated with complicated font stacks though.
Does the future look bright? Set up a spreadsheet. Define the basic core experience for legacy browsers i. When optimizing for performance we need to reflect our priorities. Load the core experience immediately, then enhancements , and then the extras. As a result, we help reduce blocking of the main thread by reducing the amount of scripts the browser needs to process.
Native JavaScript module scripts are deferred by default , so while HTML parsing is happening, the browser will download the main module. In fact, Rollup supports modules as an output format , so we can both bundle code and deploy modules in production. Parcel has module support in Parcel 2. For example, cheap Android phones in developing countries mostly run Chrome and will cut the mustard despite their limited memory and CPU capabilities.
At the moment of writing, the header is supported only in Blink it goes for client hints in general. Code-splitting is another Webpack feature that splits your codebase into "chunks" that are loaded on demand. Not all of the JavaScript has to be downloaded, parsed and compiled right away.
Once you define split points in your code, Webpack can take care of the dependencies and outputted files. It enables you to keep the initial download small and to request code on demand when requested by the application.
Alexander Kondrov has a fantastic introduction to code-splitting with Webpack and React. Watch out for prioritization issues though. Where to define split points? Umar Hansa explains how you can use Code Coverage from Devtools to achieve it. When dealing with single-page applications, we need some time to initialize the app before we can render the page. Your setting will require your custom solution, but you could watch out for modules and techniques to speed up the initial rendering time.
In general, most performance issues come from the initial time to bootstrap the app. One of the interesting ones comes from Ivan Akulov's thread. Tree-shaking will remove the variable, but not the function, because it might be used otherwise. However, if the function isn't used anywhere, you might want to remove it. Speed up your images is to serve smaller pictures on smaller screens.
With responsive-loader. Via Ivan Akulov. To reduce the negative impact to Time-to-Interactive, it might be a good idea to look into offloading heavy JavaScript into a Web Worker. Typical use cases for web workers are prefetching data and Progressive Web Apps to load and store some data in advance so that you can use it later when needed. And you could use Comlink to streamline the communication between the main page and the worker. Still some work to do, but we are getting there.
There are a few interesting case studies around web workers which show different approaches of moving framework and app logic to web workers. The conclusion: in general, there are still some challenges, but there are some good use cases already thanks, Ivan Akulov! Starting from Chrome 80, a new mode for web workers with performance benefits of JavaScript modules has been shipped, called module workers.
For most web apps, JavaScript is a better fit, and WebAssembly is best used for computationally intensive web apps , such as games. Accelerating JavaScript is a short but helpful guide that explains when to use what, and why — also with a handy flowchart and plenty of useful resources. Houssein Djirdeh and Jason Miller have recently published a comprehensive guide on how to transpile and serve modern and legacy JavaScript , going into details of making it work with Webpack and Rollup, and the tooling needed.
You can also estimate how much JavaScript you can shave off on your site or app bundles. These days we can write module-based JavaScript that runs natively in the browser, without transpilers or bundlers.
First, set up metrics that tracks if the ratio of legacy code calls is staying constant or going down, not up. You can use Puppeteer to programmatically collect code coverage. Chrome allows you to export code coverage results , too. As Andy Davies noted, you might want to collect code coverage for both modern and legacy browsers though.
After that, you set that specific image as a background on the corresponding selector in your CSS, sit back and wait for a few months if the file is going to appear in your logs. If there are no entries, nobody had that legacy component rendered on their screen: you can probably go ahead and delete it all. Check and review the polyfills that you are sending to legacy browsers and to modern browsers, and be more strategic about them.
Take a look at polyfill. Add bundle auditing into your regular workflow as well. For bundle auditing, Bundlephobia could help find the cost of adding a npm package to your bundle. You can even integrate these costs with a Lighthouse Custom Audit. This goes for frameworks, too. There are many further tools to help you make an informed decision about the impact of your dependencies and viable alternatives:. Alternatively to shipping the entire framework, you could trim your framework and compile it into a raw JavaScript bundle that does not require additional code.
Svelte does it , and so does Rawact Babel plugin which transpiles React. But there are applications that do not need all these features at initial page load. For such applications, it might make sense to use native DOM operations to build the interactive user interface.
In the article "The case of partial hydration with Next and Preact " , Lukas Bombach explains how the team behind Welt. You can also check next-super-performance GitHub repo with explanations and code snippets. Jason Miller has published working demos on how progressive hydration could be implemented with React, so you can use them right away: demo 1 , demo 2 , demo 3 also available on GitHub.
Plus, you can look into the react-prerendered-component library. As a result, here's a SPA strategy that Jeremy suggests to use for React framework but it shouldn't change significantly for other frameworks :. Hence, every interactive element is receiving a probability score for engagement, and based on that score, a client-side script decides to prefetch a resource ahead of time. You can integrate the technique to your Next. A good use case would be prefetching validation scripts required in the checkout, or speculative prefetch when a critical call-to-action comes into the viewport.
Need something less sophisticated? Quicklink , InstantClick and Instant. So is Instant. If you want to look into the science of predictive prefetching in full detail, Divya Tagtachian has a great talk on The Art of Predictive Prefetch , covering all the options from start to finish. Or perhaps even use v8-compile-cache.
When it comes to JavaScript in general, there are also some practices that are worth keeping in mind:. For security reasons, to avoid fingerprinting, browsers have been implementing partitioned caching that was introduced in Safari back in , and in Chrome last year.
So if two sites point to the exact same third-party resource URL, the code is downloaded once per domain , and the cache is "sandboxed" to that domain due to privacy implications thanks, David Calhoun!
Hence, using a public CDN will not automatically lead to better performance. Therefore, self-hosting is usually more reliable and secure, and better for performance, too. Constrain the impact of third-party scripts. The median mobile site accesses 12 third-party domains , with a median of 37 different requests or about 3 requests made to each third party.
Furthermore, these third-parties often invite fourth-party scripts to join in, ending up with a huge performance bottleneck, sometimes going as far as to the eigth-party scripts on a page.
So regularly auditing your dependencies and tag managers can bring along costly surprises. Another problem, as Yoav Weiss explained in his talk on third-party scripts , is that in many cases these scripts download resources that are dynamic. Deferring, as shown above, might be just a start though as third-party scripts also steal bandwidth and CPU time from your app.
We could be a bit more aggressive and load them only when our app has initialized. In a fantastic post on "Reducing the Site-Speed Impact of Third-Party Tags" , Andy Davies explores a strategy of minimizing the footprint of third-parties — from identifying their costs towards reducing their impact. So the first step is to identify the impact that third-parties have, by testing the site with and without scripts using WebPageTest. Preferably self-host and use a single hostname , but also use a request map to exposes fourth-party calls and detect when the scripts change.
You can use Harry Roberts' approach for auditing third parties and produce spreadsheets like this one also check Harry's auditing workflow. Afterwards, we can explore lightweight alternatives to existing scripts and slowly replace duplicates and main culprits with lighter options. Perhaps some of the scripts could be replaced with their fallback tracking pixel instead of the full tag.
The trick, then, is to load the actual embed only on interaction. One of the reasons why tag managers are usually large in size is because of the many simultaneous experiments that are running at the same time, along with many user segments, page URLs, sites etc.
And then there are anti-flicker snippets. This often results in massive delays in rendering due to massive client-side execution costs. Therefore keep track how often the anti-flicker timeout is triggered and reduce the timeout. Default blocks display of your page by up to 4s which will ruin conversion rates. Edge Computing, or Edge Slice Rerendering is always a more performant option.
Also, Christian Schaefer explores strategies for loading ads. Watch out: some third-party widgets hide themselves from auditing tools , so they might be more difficult to spot and measure. What options do we have then?
Iframes can be further constrained using the sandbox attribute, so you can disable any functionality that iframe may do, e. You could also keep third-parties in check via in-browser performance linting with feature policies , a relatively new feature that lets you opt-in or out of certain browser features on your site. As a sidenote, it could also be used to avoid oversized and unoptimized images, unsized media, sync scripts and others.
Currently supported in Blink-based browsers. As many third-party scripts are running in iframes, you probably need to be thorough in restricting their allowances. Sandboxed iframes are always a good idea , and each of the limitations can be lifted via a number of allow values on the sandbox attribute. Sandboxing is supported almost everywhere , so constrain third-party scripts to the bare minimum of what they should be allowed to do.
Consider using an Intersection Observer ; that would enable ads to be iframed while still dispatching events or getting the information that they need from the DOM e. Today , a service that groups all third-party scripts by category analytics, social, advertising, hosting, tag manager etc.
Ah, and don't forget about the usual suspects: instead of third-party widgets for sharing, we can use static social sharing buttons such as by SSBG and static links to interactive maps instead of interactive maps. In general, resources should be cacheable either for a very short time if they are likely to change or indefinitely if they are static — you can just change their version in the URL when needed. You can call it a Cache-Forever strategy , in which we could relay Cache-Control and Expires headers to the browser to only allow assets to expire in a year.
The exception are API responses e. Use Cache-control: immutable to avoid revalidation of long explicit cache lifetimes when users hit the reload button. For the reload case, immutable saves HTTP requests and improves the load time of the dynamic HTML as they no longer compete with the multitude of responses. For them, we probably want to cache as long as possible, and ensure they never get re-validated:. According to Web Almanac, "its usage has grown to 3. Do you remember the good ol' stale-while-revalidate?
When we specify the caching time with the Cache-Control response header e. This slowdown can be avoided with stale-while-revalidate ; it basically defines an extra window of time during which a cache can use a stale asset as long as it revalidates it async in the background.
Thus, it "hides" latency both in the network and on the server from clients. In June—July , Chrome and Firefox launched support of stale-while-revalidate in HTTP Cache-Control header, so as a result, it should improve subsequent page load latencies as stale assets are no longer in the critical path. Result: zero RTT for repeat views. Be wary of the vary header , especially in relation to CDNs , and watch out for the HTTP Representation Variants which help avoiding an additional round trip for validation whenever a new request differs slightly but not significantly from prior requests thanks, Guy and Mark!
Finally, keep in mind the performance cost of CORS requests in single-page applications. Note : We often assume that cached assets are retrieved instantly, but research shows that retrieving an object from cache can take hundreds of milliseconds.
In practice, it turns out that it's better to use defer instead of async. Ah, what's the difference again? According to Steve Souders , once async scripts arrive, they are executed immediately — as soon as the script is ready. If that happens very fast, for example when the script is in cache aleady, it can actually block HTML parser. Also, multiple async files will execute in a non-deterministic order.
It's worth noting that there are a few misconceptions about async and defer. In Harry Roberts' words, "If you put an async script after sync scripts, your async script is only as fast as your slowest sync script.
Also, it's not recommended to use both async and defer. Modern browsers support both, but whenever both attributes are used, async will always win. If you'd like to dive into more details, Milica Mihajlija has written a very detailed guide on Building the DOM faster , going into the details of speculative parsing, async and defer.
That threshold depends on a few things, from the type of image resource being fetched to effective connection type. But experiments conducted using Chrome on Android suggest that on 4G, However, sometimes we might need a bit more granular control. Basically, you need to create a new IntersectionObserver object, which receives a callback function and a set of options.
Then we add a target to observe. The callback function executes when the target becomes visible or invisible, so when it intercepts the viewport, you can start taking some actions before the element becomes visible.
Alejandro Garcia Anglada has published a handy tutorial on how to actually implement it, Rahul Nanwani wrote a detailed post on lazy-loading foreground and background images , and Google Fundamentals provide a detailed tutorial on lazy loading images and video with Intersection Observer as well.
Remember art-directed storytelling long reads with moving and sticky objects? You can implement performant scrollytelling with Intersection Observer , too. Check again what else you could lazy load. Even lazy-loading translation strings and emoji could help.
Opinions differ if these techniques improve user experience or not, but it definitely improves time to First Contentful Paint. These placeholders could be embedded within HTML as they naturally compress well with text compression methods. In his article, Dean Hume has described how this technique can be implemented using Intersection Observer.
And there is even a library for it. Want to go fancier? You could trace your images and use primitive shapes and edges to create a lightweight SVG placeholder, load it first, and then transition from the placeholder vector image to the loaded bitmap image. Note that content-visibility: auto; behaves like overflow: hidden; , but you can fix it by applying padding-left and padding-right instead of the default margin-left: auto; , margin-right: auto; and a declared width.
The padding basically allows elements to overflow the content-box and enter the padding-box without leaving the box model as a whole and getting cut off. Thijs Terluin has way more details about both properties and how contain-intrinsic-size is calculated by the browser, Malte Ubl shows how you can calculate it and a brief video explainer by Jake and Surma explains how it all works.
And if you need to get a bit more granular, with CSS Containment , you can manually skip layout, style and paint work for descendants of a DOM node if you need only size, alignment or computed styles on other elements — or the element is currently off-canvas. Alternatively, for offscreen images, we can display a placeholder first, and when the image is within the viewport, using IntersectionObserver, trigger a network call for the image to be downloaded in background.
Also, we can defer render until decode with img. When rendering the image, we can use fade-in animations, for example. Do you generate and serve critical CSS? Due to the limited size of packages exchanged during the slow start phase, your budget for critical CSS is around 14KB. If you go beyond that, the browser will need additional roundtrips to fetch more styles.
In our experience though, no automatic system was ever better than manual collection of critical CSS for every template, and indeed that's the approach we've moved back to recently. You can then inline critical CSS and lazy-load the rest with critters Webpack plugin.
If possible, consider using the conditional inlining approach used by the Filament Group, or convert inline code to static assets on the fly. However, for complex layouts, it might be a good idea to include the groundwork of the layout as well to avoid massive recalculation and repainting costs, hurting your Core Web Vitals score as a result. In that case, it has become common to hide non-critical content , e. It has a major downside though, as users on slow connections might never be able to read the content of the page.
Putting critical CSS and other important assets in a separate file on the root domain has benefits , sometimes even more than inlining, due to caching. That means that you could create a set of critical -CSS-files e. The catch is that server pushing was troublesome with many gotchas and race conditions across browsers.
The effect could, in fact, be negative and bloat the network buffers, preventing genuine frames in the document from being delivered. To avoid inlining on subsequent pages and instead reference the cached assets externally, we then set a cookie on the first visit to a site.
We could create one stream from multiple sources. For example, instead of serving an empty UI shell and letting JavaScript populate it, you can let the service worker construct a stream where the shell comes from a cache, but the body comes from the network. As Jeff Posnick noted , if your web app is powered by a CMS that server-renders HTML by stitching together partial templates, that model translates directly into using streaming responses, with the templating logic replicated in the service worker instead of your server.
Performance boost is quite noticeable. Browser support? And if you feel adventurous again, you can check an experimental implementation of streaming requests , which allows you to start sending the request while still generating the body. Available in Chrome In fact, you could rewrite requests for high DPI images to low DPI images , remove web fonts, fancy parallax effects, preview thumbnails and infinite scroll, turn off video autoplay, server pushes, reduce the number of displayed items and downgrade image quality, or even change how you deliver markup.
Tim Vereecke has published a very detailed article on data-s h aver strategies featuring many options for data saving. Who is using save-data , you might be wondering? With the Save-Data mode on, Chrome Mobile will provide an optimized experience, i. Save my name, email, and website in this browser for the next time I comment.
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