Performance - Repaint and reflow

Setting Height And Width On Images Is Important Again

An example layout with a title and two paragraphs, where the second paragraph has to shift down to make space for an image.

Layout shift after image loads.

tl;dr: browsers now prevent layout shifting when images load using the inline width and height values, provided that you use the correct CSS properties when making images responsive.

Layout shifts are very disrupting to the user, especially if you have already started reading the article and suddenly you are thrown off by a jolt of movement, and you have to find your place again. This also puts extra work on the browser to recalculate the page layout as each image arrives across the internet. On a complex page with a lot of images this can place a considerable load on the device at a time when it’s probably got a lot of better things to deal with!

The traditional way to avoid this was to provide width and height attributes in the <img> markup so even when the browser has just the HTML, it is still able to allocate the appropriate amount of space[:]

Code language: HTML

<h1>Your title</h1>
<p>Introductory paragraph.</p>
<img src="hero_image.jpg" alt=""
   width="400" height="400">
<p>Lorem ipsum dolor sit amet, consectetur…</p>

Then the render [should happen] like below, where the appropriate amount of space is set aside for the image when it arrives, and there is no jarring shift of the text as the image is downloaded:

An example layout mockup with a title, a paragraph, space for an image and then a second paragraph, where the text does not shift when the image loads.

Text should not shift if image dimensions are provided so appropriate space can be allocated.

Even ignoring the annoying impact to the user in content jumping around (which you shouldn’t!), the impact on the CPU [of layout shifts] can also be quite substantial.


So, once we add the dimensions and [max-width: 100%; height: auto;], we get the best of both worlds, right? No layout shifts, but also the ability to resize images using CSS? Well until very recently you might have been surprised to find out the answer was in fact: no (I was — hence why I decided to write this article).


This affects any page where we constrain the image size in a responsive manner — i.e. small screen mobile devices. These are likely to be the very users suffering with network constraints and limited processing power that will suffer most from layout shifts! Of course, we ideally should be delivering appropriately sized images for the screen size, but you cannot cover every device size, so often images will need some resizing by the browser, particularly on mobile.


[I]f the following four conditions are true, then the correct image dimensions could be calculated without needing to wait for the images to download, and so without the need of a content layout shift:

  • height is set on the element in HTML
  • width is set on the element in HTML
  • height (or width) is set in the CSS — including using percentage values like max-width: 100%;
  • width (or height) is set to auto in the CSS.

If any one of these were not set, then the calculation would not be possible, and so would fail and be ignored and have to wait for the image to be downloaded.


So instead [of waiting for a new CSS property], [browsers] could implement the equivalent logic deep in rendering code rather than exposing it via the user-agent stylesheet, but the effect is the same.


Firefox went ahead and did this as an experiment and then turned it on by default for Firefox 71. Once that was released, then your site may well have just got faster for free — thanks Mozilla!


After Firefox’s successful experimentation, Chrome also decided to implement this (again using the layout coded method for now rather than default user-agent stylesheet), and rolled it out by default in Chrome 79. This also took care of the other chromium-based browsers (Edge, Opera and Brave, for example). More recently, in January 2020, Apple added it to their Tech Preview edition of Safari, meaning it should hopefully be coming to the production version of Safari soon, and with that, the last of the major browsers will have implemented this and the web will become better and less jolty for a huge number of sites.

FastDom: eliminate layout thrashing by batching DOM measurement and mutation tasks

FastDom works as a regulatory layer between your app/library and the DOM. By batching DOM access we avoid unnecessary document reflows and dramatically speed up layout performance.

Each measure/mutate job is added to a corresponding measure/mutate queue. The queues are emptied (reads, then writes) at the turn of the next frame using window.requestAnimationFrame.

FastDom aims to behave like a singleton across all modules in your app. When any module requires 'fastdom' they get the same instance back, meaning FastDom can harmonize DOM access app-wide.


Creates a new empty DocumentFragment into which DOM nodes can be added to build an offscreen DOM tree.


DocumentFragments are DOM Node objects which are never part of the main DOM tree. The usual use case is to create the document fragment, append elements to the document fragment and then append the document fragment to the DOM tree. In the DOM tree, the document fragment is replaced by all its children.

Since the document fragment is in memory and not part of the main DOM tree, appending children to it does not cause page reflow (computation of element’s position and geometry). Historically, using document fragments could result in better performance.

Code language: JavaScript

var element  = document.getElementById('ul'); // assuming ul exists
var fragment = document.createDocumentFragment();
var browsers = ['Firefox', 'Chrome', 'Opera', 
    'Safari', 'Internet Explorer'];
browsers.forEach(function(browser) {
    var li = document.createElement('li');
    li.textContent = browser;

Taming huge collections of DOM nodes

Hajime Yamasaki Vukelic has come to the conclusion that, if you’re dealing with a really big number of DOM nodes that need to be updated in real time, frameworks are usually incredibly slow on top of the already-slow DOM operations you have to do. The solution they settled on is to avoid frameworks altogether for these scenarios and use vanilla JavaScript. Also of note are that repaints and reflows are going to be big bottlenecks regardless of what you use.

  • If you are looking for performance, don’t use frameworks. Period.
  • At the end of the day, DOM is slow.
  • Repaints and reflows are even slower.
  • Whatever performance you get out of your app, repaints and reflows are still going to be the last remaining bottleneck.
  • Keep the number of DOM nodes down.
  • Cache created DOM nodes, and use them as a pool of pre-assembled elements you can put back in the page as needed.
  • Logging the timings in IE/Edge console is unreliable because the developer tools have a noticeable performance hit.
  • Measure! Always measure performance first, then only fix the issues you’ve reliably identified.