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 search engine 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 Structure Shift metric is inflicting hassle 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 photographs, 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 lined that already in a previous article. Nor will I speak 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 just a little background wanted to know among the methods.

Why CLS Is Completely different

CLS is, for my part, 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 methods and methods of considering to aim to optimize it. It’s a really completely different beast to the opposite two Core Internet Vitals.

Wanting briefly on the different two Core Internet Vitals, Largest Contentful Paint (LCP) does precisely as its title suggests and is extra of a twist on earlier loading metrics that measures how shortly the web page hundreds. 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to have a look at the loading velocity of the most related content material, nevertheless it’s principally reusing the outdated methods of making certain that the content material hundreds as shortly as attainable. The right way to optimize your LCP needs to be a comparatively well-understood drawback for many internet 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 decreasing!) your JavaScript and is often site-specific. That’s to not say fixing points with these two metrics are simple, however they’re moderately well-understood issues.

One purpose that CLS is completely different is that it’s measured by the lifetime of the web page — that’s the “cumulative” a part of the title! The opposite two Core Internet Vitals cease after the primary part is discovered on the web page after load (for LCP), or for the primary interplay (for FID). Because of this our conventional lab-based instruments, like Lighthouse, typically don’t absolutely replicate 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 would like 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 selected 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 seem, for instance by increasing a collapsed part.

CLS is about measuring surprising shifts. Scrolling shouldn’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 think about 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. Moderately than accumulating shifts over the entire web page time to calculate the CLS rating like has been completed up till now, the rating shall be calculated primarily based on the most important set of shifts inside a selected timeboxed window.

This implies tha in case you have three chunks of CLS of 0.05, 0.06, and 0.04 then beforehand this could have been recorded as 0.15 (i.e. over the “good” restrict of 0.1), whereas now shall 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 timeframe (i.e. if that 0.06 CLS rating was brought on by three separate shifts of 0.02), nevertheless it’s simply not cumulative over the full lifetime of the web page anymore.

Saying that, in the event you 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 taking a look at 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 transient introduction to among the methodology about CLS, let’s transfer on to among the options! All of those methods principally contain setting apart the correct quantity of area 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 obtainable to internet 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 peak attributes set in your photographs. With out them, a picture will trigger the following content material to shift to make approach 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.
Structure shift after picture hundreds. (Large preview)

That is merely a matter of fixing 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" peak="400">

You will discover the size of the picture by opening DevTools and hovering over (or tapping by) the ingredient.

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 Measurement (which is the precise measurement of the picture supply) and the browser will then scale these all the way down to the rendered measurement if you use CSS to vary these.

Fast Tip: If, like me, you’ll be able to’t bear in mind whether or not it’s width and peak or peak and width, consider it as X and Y coordinates so, like X, width is all the time given first.

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

img {
  max-width: 100%;
  peak: 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 photographs (set the width and peak on the fallback img ingredient), although not but for photographs of various aspect-ratios — it’s being worked on, and till then you must nonetheless set width and peak as any values shall be higher than the 0 by 0 defaults!

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

The New aspect-ratio CSS Property

The width and peak method above, to calculate the peak for responsive photographs, 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 it will likely be coming to the secure model quickly.

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

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

Curiously, 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 structure shift right here.

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

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

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

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

Chrome is the one browser that feeds again CLS to Google and it helps aspect-ratio that means that can clear up your CLS points by way of 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 signifies that I might say we’re on the level the place we will 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 measurement however a hard and fast peak as a substitute, think about using min-height. This may very well be for a fastened peak header, for instance and we will have completely different headings for the completely different break-points utilizing media queries as normal:

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

After all the identical applies to min-width for horizontally positioned parts, nevertheless 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, in the event you 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 can use the next snippet to go away the area for extra content material when the main-content div is rendered initially.

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

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

Alternatively, you should use the ::earlier than pseudo-element so as to add the area to keep away from the shift on the main-content ingredient as effectively:

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

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

Verify Fallback Parts

I like to make use of progressive enhancement to offer a fundamental web site, even with out JavaScript the place attainable. 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 as a result of “Desk of Contents” menu button within the header. Earlier than the JavaScript kicks in it is a easy hyperlink, styled to seem 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 can help 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 (prime), after which enhanced with JavaScript to be a dynamic menu (backside). (Large preview)

I used semantic parts and so used an anchor ingredient (<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 all the web page down a few pixels. So this had fairly an affect 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 carry the 2 parts into sync (it might even have been remediated by setting a min-height on the header as mentioned above), nevertheless it confused me for a bit. I’m positive I’m not the one one to have made this error so pay attention to 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 Structure Shifts

Internet fonts are one other widespread reason behind CLS as a result of browser initially calculating the area wanted primarily based on the fallback font, after which recalculating it when the online font is downloaded. Normally, 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 internet fonts. (Large preview)

Sadly even preloading the webfonts received’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 most often so doesn’t keep away from CLS. Saying that, if an online 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’ll be able to prefetch them.

To keep away from font-induced structure shifts altogether, we might after all not use internet fonts in any respect — together with utilizing system fonts as a substitute, or utilizing font-display: elective 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 could shift a bit, the content material beneath it received’t be pushed down even when this occurs. For instance, setting a min-height on the <h1> ingredient might stop the entire article from shifting down if barely taller fonts load in — offering the completely different fonts don’t trigger a unique variety of strains. It will scale back the affect of the shifts, nevertheless, for a lot of use-cases (e.g. generic paragraphs) it will likely be troublesome to generalize a minimal peak.

What I’m most enthusiastic about to unravel this challenge, are the new CSS Font Descriptors which let you extra simply alter 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 sophisticated, however this feature due out very quickly could lastly give us a neater resolution that’s extra more likely to achieve traction. See my previous article on this subject for extra particulars on this upcoming innovation and extra assets on that.

Preliminary Templates For Consumer-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 simple for these preliminary templates to get out of sync with the JavaScript model as new elements 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 elements are injected by JavaScript.

So overview 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 methods 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 peak initially earlier than the preliminary template is even inserted.

<div id="app" model="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 barely measured when it’s within the viewport and so impacts the person. By default, an empty div has a peak of 0px, so give it a min-height that’s nearer to what the precise peak shall be when the app hundreds.

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 in the event you 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 prime 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 on account of latest enter. (Large preview)

You will note a filmstrip of the web page during which I loaded among the feedback on one other Smashing Journal article so within the half I’ve circled, you’ll be able to nearly make out the feedback loading and the crimson 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’ll get extra element within the Abstract tab beneath.

Right here you’ll be able to see that we acquired a huge 0.3359 rating — effectively 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 ready 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 circumstances 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 is probably not added to the web page till after the five hundred ms restrict, so CLS is dangerous.

Ideally, the entire interplay shall be completed inside 500ms, however you are able to do some issues to put aside the required area utilizing the methods above whereas that processing is happening in order that if it does take greater than the magic 500 ms, then you definitely’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 affect CLS. Whereas this might sound a bit restrictive, the purpose 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 in the event you disagree, or have a use case they may not have thought of, then the Chrome group is open to feedback on this.

Synchronous JavaScript

The ultimate method I’m going to debate is just a little controversial because it goes in opposition to well-known internet efficiency recommendation, however it may be the one methodology in sure conditions. Principally, in case you have content material that goes to trigger shifts, then one resolution to keep away from the shifts is to not render it till it’s settled down!

The beneath HTML will disguise 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 shall be rendered (together with the second model block to unhide it) and so no shifts shall 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 system also can even be utilized with exterior JavaScript, however this may 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 obtainable 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 positive will make some internet efficiency folks cringe, as recommendation is to make use of async, defer or the newer sort="module" (that are defer-ed by default) on JavaScript particularly to keep away from blocking render, whereas we’re doing the other right here! Nevertheless, if content material can’t be predetermined and it’ll 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 prime 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 prime 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 numerous methods to keep away from CLS. For instance by having them on the backside of the web page, or overlaying them on prime of the content material, quite than shifting the content material down. We most well-liked to maintain the content material on the prime 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 house owners could favor to be on the prime 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 “major” and “apart” columns (for causes I received’t go into, it was not attainable 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 routinely unhidden even when the JavaScript doesn’t run for some purpose and the unshifted content material is proven.

Utilizing this system can affect different metrics (significantly 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 think about for these circumstances the place no different possibility exists.


Cumulative Structure 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 publish, we’ve mentioned varied suggestions and tips to keep away from this. I’m glad the highlight the Core Internet Vitals have shone on this irritating challenge — for too lengthy we internet builders (and I undoubtedly embrace myself on this) have ignored this drawback.

Cleansing up my very own web sites has led to a greater expertise for all guests. I encourage you to take a look at your CLS points too, and hopefully a few of these suggestions shall 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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Smashing Editorial
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