Wednesday June 2, 2021 By David Quintanilla
How To Fix Cumulative Layout Shift (CLS) Issues — Smashing Magazine

Google’s Core Internet Vitals initiative has taken the web optimization and Internet Efficiency worlds by storm and lots of websites are busy optimizing their Web page Expertise to maximise the rating issue. The Cumulative Format Shift metric is inflicting bother to quite a lot of websites, so let’s take a look at methods of addressing any points for that metric.

Cumulative Layout Shift (CLS) makes an attempt to measure these jarring actions of the web page as new content material — be it photos, ads, or no matter — comes into play later than the remainder of the web page. It calculates a rating primarily based on how a lot of the web page is unexpectedly shifting about, and the way typically. These shifts of content material are very annoying, making you lose your home in an article you’ve began studying or, worse nonetheless, making you click on on the unsuitable button!

On this article, I’m going to debate some front-end patterns to scale back CLS. I’m not going to speak an excessive amount of about measuring CLS as I’ve coated that already in a previous article. Nor will I discuss an excessive amount of concerning the mechanics of how CLS is calculated: Google has some good documentation on that, and Jess Peck’s The Almost-Complete Guide to Cumulative Layout Shift is an superior deep dive into that too. Nevertheless, I’ll give slightly background wanted to know a few of the strategies.

Why CLS Is Totally different

CLS is, in my view, probably the most attention-grabbing of the Core Internet Vitals, partially as a result of it’s one thing we’ve by no means actually measured or optimized for earlier than. So, it typically requires new strategies and methods of pondering to try to optimize it. It’s a really completely different beast to the opposite two Core Internet Vitals.

Trying briefly on the different two Core Internet Vitals, Largest Contentful Paint (LCP) does precisely as its identify suggests and is extra of a twist on earlier loading metrics that measures how shortly the web page masses. Sure, we’ve modified how we outlined the person expertise of the web page load to have a look at the loading velocity of the most related content material, however it’s principally reusing the previous strategies of making certain that the content material masses as shortly as doable. Learn how to optimize your LCP needs to be a comparatively well-understood downside for many net pages.

First Input Delay (FID) measures any delays in interactions and seems not to be a problem for most sites. Optimizing that’s often a matter of cleansing up (or lowering!) your JavaScript and is often site-specific. That’s to not say fixing points with these two metrics are straightforward, however they’re moderately well-understood issues.

One purpose that CLS is completely different is that it’s measured by way of the lifetime of the web page — that’s the “cumulative” a part of the identify! The opposite two Core Internet Vitals cease after the principle part is discovered on the web page after load (for LCP), or for the primary interplay (for FID). Which means that our conventional lab-based instruments, like Lighthouse, typically don’t totally mirror the CLS as they calculate solely the preliminary load CLS. In actual life, a person will scroll down the web page and will get extra content material dropping in inflicting extra shifts.

CLS can be a little bit of a synthetic quantity that’s calculated primarily based on how a lot of the web page is shifting about and the way typically. Whereas LCP and FID are measured in milliseconds, CLS is a unitless quantity output by a complex calculation. We wish the web page to be 0.1 or underneath to move this Core Internet Important. Something above 0.25 is seen as “poor”.

Shifts brought on by person interplay are not counted. That is outlined as inside 500ms of a particular set of person interactions although pointer events and scroll are excluded. It’s presumed {that a} person clicking on a button would possibly anticipate content material to look, for instance by increasing a collapsed part.

CLS is about measuring sudden shifts. Scrolling mustn’t trigger content material to maneuver round if a web page is constructed optimally, and equally hovering over a product picture to get a zoomed-in model for instance also needs to not trigger the opposite content material to leap about. However there are after all exceptions and people websites want to contemplate how you can react to this.

CLS can be continually evolving with tweaks and bug fixes. It has simply had an even bigger change introduced that ought to give some respite to long-lived pages, like Single Web page Apps (SPA) and infinite scrolling pages, which many felt have been unfairly penalized in CLS. Fairly than accumulating shifts over the entire web page time to calculate the CLS rating like has been achieved up till now, the rating will likely be calculated primarily based on the most important set of shifts inside a particular timeboxed window.

This implies tha in case you have three chunks of CLS of 0.05, 0.06, and 0.04 then beforehand this may have been recorded as 0.15 (i.e. over the “good” restrict of 0.1), whereas now will likely be scored as 0.06. It’s nonetheless cumulative within the sense that the rating could also be made up of separate shifts inside that time-frame (i.e. if that 0.06 CLS rating was brought on by three separate shifts of 0.02), however it’s simply not cumulative over the overall lifetime of the web page anymore.

Saying that, if you happen to clear up the causes of that 0.06 shift, then your CLS will then be reported because the subsequent largest one (0.05) so it nonetheless is all of the shifts over the lifetime of the web page — it’s simply selecting to report solely the most important one because the CLS rating.

With that temporary introduction to a few of the methodology about CLS, let’s transfer on to a few of the options! All of those strategies principally contain setting apart the correct quantity of house earlier than extra content material is loaded — whether or not that’s media or JavaScript-injected content material, however there’s a number of completely different choices out there to net builders to do that.

Set Width And Heights On Photos And iFrames

I’ve written about this before, however one of many best issues you are able to do to scale back CLS is to make sure you have width and top attributes set in your photos. With out them, a picture will trigger the next content material to shift to make method for it after it downloads:

An example layout with a title and two paragraphs, where the second paragraph has to shift down to make space for an image.
Format shift after picture masses. (Large preview)

That is merely a matter of adjusting your picture markup from:

<img src="https://smashingmagazine.com/2021/06/how-to-fix-cumulative-layout-shift-issues/hero_image.jpg" alt="...">


<img src="https://smashingmagazine.com/2021/06/how-to-fix-cumulative-layout-shift-issues/hero_image.jpg" alt="..."
   width="400" top="400">

You’ll find the scale of the picture by opening DevTools and hovering over (or tapping by way of) the component.

Chrome Dev Tools screenshot showing the image, rendered size, rendered aspect ratio, intrinsic size, intrinsic aspect ratio, file size and current source.
Chrome DevTools exhibits the picture dimensions and side ratios when hovering over a component. (Large preview)

I counsel utilizing the Intrinsic Dimension (which is the precise dimension of the picture supply) and the browser will then scale these all the way down to the rendered dimension if you use CSS to alter these.

Fast Tip: If, like me, you may’t keep in mind whether or not it’s width and top or top and width, consider it as X and Y coordinates so, like X, width is at all times given first.

If in case you have responsive photos and use CSS to alter the picture dimensions (e.g. to constrain it to a max-width of 100% of the display screen dimension), then these attributes can be utilized to calculate the top — offering you keep in mind to override this to auto in your CSS:

img {
  max-width: 100%;
  top: auto;

All modern browsers support this now, although didn’t till lately as covered in my article. This additionally works for <image> parts and srcset photos (set the width and top on the fallback img component), although not but for photos of various aspect-ratios — it’s being worked on, and till then it’s best to nonetheless set width and top as any values will likely be higher than the 0 by 0 defaults!

This additionally works on native lazy-loaded photos (although Safari doesn’t help native lazy loading by default but).

The New aspect-ratio CSS Property

The width and top method above, to calculate the peak for responsive photos, will be generalized to different parts utilizing the brand new CSS aspect-ratio property, which is now supported by Chromium-based browsers and Firefox, however can be in Safari Know-how Preview so hopefully meaning will probably be coming to the steady model quickly.

So you possibly can apply it to an embedded video for instance in 16:9 ratio:

video {
  max-width: 100%;
  top: auto;
  aspect-ratio: 16 / 9;
<video controls width="1600" top="900" poster="...">
    <supply src="https://smashingmagazine.com/media/video.webm"
    <supply src="/media/video.mp4"
    Sorry, your browser would not help embedded movies.

Apparently, with out defining the aspect-ratio property, browsers will ignore the height for responsive video elements and use a default aspect-ratio of 2:1, so the above is required to keep away from a format shift right here.

Sooner or later, it ought to even be doable to set the aspect-ratio dynamically primarily based on the component attributes by utilizing aspect-ratio: attr(width) / attr(top); however sadly this isn’t supported but.

Or you may even use aspect-ratio on a <div> component for some kind of {custom} management you’re creating to make it responsive:

#my-square-custom-control {
  max-width: 100%;
  top: auto;
  width: 500px;
  aspect-ratio: 1;
<div id="my-square-custom-control"></div>

For these browsers that don’t help aspect-ratio you should use the older padding-bottom hack however, with the simplicity of the newer aspect-ratio and vast help (particularly as soon as this strikes from Safari Technical Preview to common Safari), it’s laborious to justify that older technique.

Chrome is the one browser that feeds again CLS to Google and it helps aspect-ratio that means that may clear up your CLS points when it comes to Core Internet Vitals. I don’t like prioritizing the metrics over the customers, however the truth that the opposite Chromium and Firefox browsers have this and Safari will hopefully quickly, and that it is a progressive enhancement implies that I might say we’re on the level the place we are able to go away the padding-bottom hack behind and write cleaner code.

Make Liberal Use Of min-height

For these parts that don’t want a responsive dimension however a set top as a substitute, think about using min-height. This may very well be for a fastened top header, for instance and we are able to have completely different headings for the completely different break-points utilizing media queries as ordinary:

header {
  min-height: 50px;
@media (min-width: 600px) {
  header {
    min-height: 200px;

In fact the identical applies to min-width for horizontally positioned parts, however it’s usually the peak that causes the CLS points.

A extra superior method for injected content material and superior CSS selectors is to focus on when anticipated content material has not been inserted but. For instance, if you happen to had the next content material:

<div class="container">
  <div class="main-content">...</div>

And an additional div is inserted by way of JavaScript:

<div class="container">
  <div class="additional-content">.../div>
  <div class="main-content">...</div>

Then you possibly can use the next snippet to go away the house for added content material when the main-content div is rendered initially.

.main-content:first-child {
   margin-top: 20px; 

This code will really create a shift to the main-content component because the margin counts as a part of that component so it’s going to seem to shift when that’s eliminated (regardless that it doesn’t really transfer on display screen). Nevertheless, not less than the content material beneath it is not going to be shifted so ought to cut back CLS.

Alternatively, you should use the ::earlier than pseudo-element so as to add the house to keep away from the shift on the main-content component as nicely:

.main-content:first-child::earlier than {
   content material: '';
   min-height: 20px;
   show: block;

However in all honesty, the higher answer is to have the div within the HTML and make use of min-height on that.

Test Fallback Parts

I like to make use of progressive enhancement to offer a fundamental web site, even with out JavaScript the place doable. Sadly, this caught me out lately on one web site I keep when the fallback non-JavaScript model was completely different than when the JavaScript kicked in.

The difficulty was because of the “Desk of Contents” menu button within the header. Earlier than the JavaScript kicks in it is a easy hyperlink, styled to appear like the button that takes you to the Desk of Contents web page. As soon as JavaScript kicks in, it turns into a dynamic menu to will let you navigate on to no matter web page you need to go to from that web page.

Screenshots of two Table of Contents navigation components styled like a button. With JavaScript this opens a dynamic menu as shown in the second image.
A Desk of Contents header part which is initially rendered as a easy hyperlink (high), after which enhanced with JavaScript to be a dynamic menu (backside). (Large preview)

I used semantic parts and so used an anchor component (<a href="#table-of-contents">) for the fallback hyperlink however changed that with a <button> for the JavaScript-driven dynamic menu. These have been styled to look the identical, however the fallback hyperlink was a few pixels smaller than the button!

This was so small, and the JavaScript often kicked in so shortly, that I had not seen it was off. Nevertheless, Chrome seen it when calculating the CLS and, as this was within the header, it shifted the complete web page down a few pixels. So this had fairly an impression on the CLS rating — sufficient to knock all our pages into the “Wants Enchancment” class.

This was an error on my half, and the repair was merely to convey the 2 parts into sync (it may even have been remediated by setting a min-height on the header as mentioned above), however it confused me for a bit. I’m certain I’m not the one one to have made this error so concentrate on how the web page renders with out JavaScript. Don’t assume your customers disable JavaScript? All your users are non-JS while they’re downloading your JS.

Internet Fonts Trigger Format Shifts

Internet fonts are one other widespread explanation for CLS because of the browser initially calculating the house wanted primarily based on the fallback font, after which recalculating it when the online font is downloaded. Often, the CLS is small, offering a equally sized fallback font is used, so typically they don’t trigger sufficient of an issue to fail Core Internet Vitals, however they are often jarring for customers nonetheless.

Two screenshots of a Smashing Magazine article with different fonts. The text is noticeably different sized and an extra sentence can fit in when the web fonts are used.
Smashing Journal article with fallback font and with full net fonts. (Large preview)

Sadly even preloading the webfonts gained’t assist right here as, whereas that reduces the time the fallback fonts are used for (so is nice for loading efficiency — LCP), it nonetheless takes time to fetch them, and so the fallbacks will nonetheless be utilized by the browser typically so doesn’t keep away from CLS. Saying that, if you recognize an internet font is required on the following web page (say you’re on a login web page and know the following web page makes use of a particular font) then you may prefetch them.

To keep away from font-induced format shifts altogether, we may after all not use net fonts in any respect — together with utilizing system fonts as a substitute, or utilizing font-display: optionally available to not use them if not downloaded in time for the preliminary render. However neither of these are very passable, to be trustworthy.

Another choice is to make sure the sections are appropriately sized (e.g. with min-height) so whereas the textual content in them might shift a bit, the content material beneath it gained’t be pushed down even when this occurs. For instance, setting a min-height on the <h1> component may stop the entire article from shifting down if barely taller fonts load in — offering the completely different fonts don’t trigger a special variety of traces. It will cut back the impression of the shifts, nevertheless, for a lot of use-cases (e.g. generic paragraphs) will probably be troublesome to generalize a minimal top.

What I’m most enthusiastic about to unravel this situation, are the new CSS Font Descriptors which let you extra simply regulate fallback fonts in CSS:

@font-face {
  font-family: 'Lato';
  src: url('/static/fonts/Lato.woff2') format('woff2');
  font-weight: 400;

@font-face {
    font-family: "Lato-fallback";
    size-adjust: 97.38%;
    ascent-override: 99%;
    src: native("Arial");

h1 {
    font-family: Lato, Lato-fallback, sans-serif;

Prior to those, adjusting the fallback font required utilizing the Font Loading API in JavaScript which was extra difficult, however this feature due out very quickly might lastly give us a neater answer that’s extra more likely to acquire traction. See my previous article on this subject for extra particulars on this upcoming innovation and extra assets on that.

Preliminary Templates For Shopper-side Rendered Pages

Many client-side rendered pages, or Single Web page Apps, render an preliminary fundamental web page utilizing simply HTML and CSS, after which “hydrate” the template after the JavaScript downloads and executes.

It’s straightforward for these preliminary templates to get out of sync with the JavaScript model as new parts and options are added to the app within the JavaScript however not added to the preliminary HTML template which is rendered first. This then causes CLS when these parts are injected by JavaScript.

So assessment all of your preliminary templates to make sure they’re nonetheless good preliminary placeholders. And if the preliminary template consists of empty <div>s, then use the strategies above to make sure they’re sized appropriately to attempt to keep away from any shifts.

Moreover, the preliminary div which is injected with the app ought to have a min-height to keep away from it being rendered with 0 top initially earlier than the preliminary template is even inserted.

<div id="app" fashion="min-height:900px;"></div>

So long as the min-height is bigger than most viewports, this could keep away from any CLS for the web site footer, for instance. CLS is simply measured when it’s within the viewport and so impacts the person. By default, an empty div has a top of 0px, so give it a min-height that’s nearer to what the precise top will likely be when the app masses.

Guarantee Consumer Interactions Full Inside 500ms

Consumer interactions that trigger content material to shift are excluded from CLS scores. These are restricted to 500 ms after the interplay. So if you happen to click on on a button, and do some complicated processing that takes over 500 ms after which render some new content material, then your CLS rating goes to endure.

You’ll be able to see if the shift was excluded in Chrome DevTools by utilizing the Efficiency tab to file the web page after which discovering the shifts as proven within the subsequent screenshot. Open DevTools go to the very intimidating (however very helpful when you get a dangle of it!) Efficiency tab after which click on on the file button within the high left (circled on the picture beneath) and work together along with your web page, and cease recording as soon as full.

Screenshot of Chrome Dev Tools with a shift selected and the Summary of this shows that it had recent input and so the shift is not included in the Cumulative Score.
Utilizing the Efficiency tab in Chrome Dev Instruments to see if shifts are excluded as a consequence of latest enter. (Large preview)

You will note a filmstrip of the web page through which I loaded a few of the feedback on one other Smashing Journal article so within the half I’ve circled, you may nearly make out the feedback loading and the purple footer being shifted down offscreen. Additional down the Efficiency tab, underneath the Expertise line, Chrome will put a reddish-pinkish field for every shift and if you click on on that you’re going to get extra element within the Abstract tab beneath.

Right here you may see that we bought a large 0.3359 rating — nicely previous the 0.1 threshold we’re aiming to be underneath, however the Cumulative rating has not included this, as a result of Had latest enter is about to Makes use of.

Making certain interactions solely shift content material inside 500 ms borders on what First Enter Delay makes an attempt to measure, however there are instances when the person may even see that the enter had an impact (e.g. a loading spinner is proven) so FID is nice, however the content material will not be added to the web page till after the five hundred ms restrict, so CLS is unhealthy.

Ideally, the entire interplay will likely be completed inside 500ms, however you are able to do some issues to put aside the mandatory house utilizing the strategies above whereas that processing is occurring in order that if it does take greater than the magic 500 ms, then you definately’ve already dealt with the shift and so is not going to be penalized for it. That is particularly helpful when fetching content material from the community which may very well be variable and outdoors your management.

Different gadgets to be careful for are animations that take longer than 500ms and so can impression CLS. Whereas this might sound a bit restrictive, the goal of CLS isn’t to restrict the “enjoyable”, however to set affordable expectations of person expertise and I don’t assume it’s unrealistic to anticipate these to take 500ms or underneath. However if you happen to disagree, or have a use case they won’t have thought of, then the Chrome crew is open to feedback on this.

Synchronous JavaScript

The ultimate method I’m going to debate is slightly controversial because it goes towards well-known net efficiency recommendation, however it may be the one technique in sure conditions. Principally, in case you have content material that you recognize goes to trigger shifts, then one answer to keep away from the shifts is to not render it till it’s settled down!

The beneath HTML will conceal the div initially, then load some render-blocking JavaScript to populate the div, then unhide it. Because the JavaScript is render-blocking nothing beneath this will likely be rendered (together with the second fashion block to unhide it) and so no shifts will likely be incurred.

.cls-inducing-div {
    show: none;

<div class="cls-inducing-div"></div>

.cls-inducing-div {
    show: block;

You will need to inline the CSS within the HTML with this system, so it’s utilized so as. The choice is to unhide the content material with JavaScript itself, however what I like concerning the above method is that it nonetheless unhides the content material even when the JavaScript fails or is turned off by the browser.

This method may even be utilized with exterior JavaScript, however this can trigger extra delay than an inline script because the exterior JavaScript is requested and downloaded. That delay will be minimized by preloading the JavaScript useful resource so it’s out there faster as soon as the parser reaches that part of code:

<hyperlink rel="preload" href="https://smashingmagazine.com/2021/06/how-to-fix-cumulative-layout-shift-issues/cls-inducing-javascript.js" as="script">
.cls-inducing-div {
    show: none;
<div class="cls-inducing-div"></div>
<script src="https://smashingmagazine.com/2021/06/how-to-fix-cumulative-layout-shift-issues/cls-inducing-javascript.js"></script>
.cls-inducing-div {
    show: block;

Now, as I say, this I’m certain will make some net efficiency folks cringe, as recommendation is to make use of async, defer or the newer kind="module" (that are defer-ed by default) on JavaScript particularly to keep away from blocking render, whereas we’re doing the alternative right here! Nevertheless, if content material can’t be predetermined and it’s going to trigger jarring shifts, then there may be little level in rendering it early.

I used this system for a cookie banner that loaded on the high of the web page and shifted content material downwards:

A screenshot of a web page, where the content is shifted down when a cookie banner is added to the top of the page.
A high of web page cookie discover or different banner can shift content material down. (Large preview)

This required studying a cookie to see whether or not to show the cookie banner or not and, whereas that may very well be accomplished server-side, this was a static web site with no means to dynamically alter the returned HTML.

Cookie banners will be carried out in several methods to keep away from CLS. For instance by having them on the backside of the web page, or overlaying them on high of the content material, moderately than shifting the content material down. We most popular to maintain the content material on the high of the web page, so had to make use of this system to keep away from the shifts. There are numerous different alerts and banners that web site homeowners might desire to be on the high of the web page for varied causes.

I additionally used this system on one other web page the place JavaScript strikes content material round into “essential” and “apart” columns (for causes I gained’t go into, it was not doable to assemble this correctly in HTML server-side). Once more hiding the content material, till the JavaScript had rearranged the content material, and solely then displaying it, averted the CLS points that have been dragging these pages’ CLS rating down. And once more the content material is mechanically unhidden even when the JavaScript doesn’t run for some purpose and the unshifted content material is proven.

Utilizing this system can impression different metrics (notably LCP and likewise First Contentful Paint) as you’re delaying rendering, and likewise probably blocking browsers’ look forward preloader, however it’s one other device to contemplate for these instances the place no different choice exists.


Cumulative Format Shift is brought on by content material altering dimensions, or new content material being injected into the web page by late operating JavaScript. On this submit, we’ve mentioned varied ideas and tips to keep away from this. I’m glad the highlight the Core Internet Vitals have shone on this irritating situation — for too lengthy we net builders (and I undoubtedly embrace myself on this) have ignored this downside.

Cleansing up my very own web sites has led to a greater expertise for all guests. I encourage you to have a look at your CLS points too, and hopefully a few of these ideas will likely be helpful if you do. Who is aware of, it’s possible you’ll even handle to get all the way down to the elusive 0 CLS score for all of your pages!

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