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 marketing 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 a whole 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 house in an article you’ve began studying or, worse nonetheless, making you click on on the fallacious button!

On this article, I’m going to debate some front-end patterns to cut 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 speak an excessive amount of in regards to 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 grasp among the strategies.

Why CLS Is Totally different

CLS is, in my view, probably the most fascinating 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 considering to aim to optimize it. It’s a really totally 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 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 consumer expertise of the web page load to take a look at the loading velocity of the most related content material, but it surely’s principally reusing the previous strategies of guaranteeing that the content material masses as shortly as doable. Find out how to optimize your LCP ought 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 normally a matter of cleansing up (or lowering!) your JavaScript and is normally site-specific. That’s to not say fixing points with these two metrics are simple, however they’re fairly well-understood issues.

One cause that CLS is totally 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 replicate the CLS as they calculate solely the preliminary load CLS. In actual life, a consumer will scroll down the web page and should get extra content material dropping in inflicting extra shifts.

CLS can be a little bit of a man-made 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 below to cross this Core Internet Important. Something above 0.25 is seen as “poor”.

Shifts brought on by consumer interplay are not counted. That is outlined as inside 500ms of a selected set of consumer interactions although pointer events and scroll are excluded. It’s presumed {that a} consumer clicking on a button may anticipate content material to look, 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 in fact exceptions and people websites want to think about the right way to react to this.

CLS can be continually evolving with tweaks and bug fixes. It has simply had a much 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 had been unfairly penalized in CLS. Relatively than accumulating shifts over the entire web page time to calculate the CLS rating like has been carried out up till now, the rating will probably be calculated primarily based on the biggest 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 might have been recorded as 0.15 (i.e. over the “good” restrict of 0.1), whereas now will probably 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), but it surely’s simply not cumulative over the whole lifetime of the web page anymore.

Saying that, if you happen to resolve 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 biggest 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 strategies principally contain setting apart the right amount of area earlier than further content material is loaded — whether or not that’s media or JavaScript-injected content material, however there’s a number of totally 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 cut 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 manner 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 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" peak="400">

Yow will discover the size 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 reveals the picture dimensions and facet 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 right down to the rendered measurement while you use CSS to vary these.

Fast Tip: If, like me, you may’t keep 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 at all times given first.

When you’ve got responsive photographs and use CSS to vary the picture dimensions (e.g. to constrain it to a max-width of 100% of the display measurement), then these attributes can be utilized to calculate the peak — offering you keep 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 just lately as covered in my article. This additionally works for <image> components and srcset photographs (set the width and peak on the fallback img component), although not but for photographs of various aspect-ratios — it’s being worked on, and till then it is best to nonetheless set width and peak as any values will probably 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 approach above, to calculate the peak for responsive photographs, will be generalized to different components utilizing the brand new CSS aspect-ratio property, which is now supported by Chromium-based browsers and Firefox, however can be in Safari Expertise Preview so hopefully which means will probably be coming to the steady model quickly.

So you could possibly 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 doable to set the aspect-ratio dynamically primarily based on the component attributes through the use of aspect-ratio: attr(width) / attr(peak); however sadly this isn’t supported but.

Or you may even use aspect-ratio on a <div> component for some form of {custom} management you might be 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 broad assist (particularly as soon as this strikes from Safari Technical Preview to common Safari), it’s arduous to justify that older methodology.

Chrome is the one browser that feeds again CLS to Google and it helps aspect-ratio that means that may resolve 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 are able to go away the padding-bottom hack behind and write cleaner code.

Make Liberal Use Of min-height

For these components that don’t want a responsive measurement however a set peak as an alternative, think about using min-height. This might be for a mounted peak header, for instance and we are able to have totally different headings for the totally different break-points utilizing media queries as regular:

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

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

A extra superior approach 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 could possibly 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 component because the margin counts as a part of that component so it is going to seem to shift when that’s eliminated (regardless that it doesn’t truly transfer on display). Nevertheless, not less than the content material beneath it won’t be shifted so ought to cut 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 component 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 supply a fundamental web site, even with out JavaScript the place doable. Sadly, this caught me out just lately on one website I keep when the fallback non-JavaScript model was totally different than when the JavaScript kicked in.

The problem 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 appear to be 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 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 (high), after which enhanced with JavaScript to be a dynamic menu (backside). (Large preview)

I used semantic components 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 had 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 normally kicked in so shortly, that I had not observed it was off. Nevertheless, Chrome observed 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 influence 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 components into sync (it may even have been remediated by setting a min-height on the header as mentioned above), but it surely 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 suppose 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 explanation for CLS as a result of browser initially calculating the area wanted primarily based on the fallback font, after which recalculating it when the net 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 received’t assist right here as, whereas that reduces the time the fallback fonts are used for (so is sweet for loading efficiency — LCP), it nonetheless takes time to fetch them, and so the fallbacks will nonetheless be utilized by the browser normally so doesn’t keep away from CLS. Saying that, if 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 structure shifts altogether, we may in fact not use net fonts in any respect — together with utilizing system fonts as an alternative, 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.

An alternative 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 under it received’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 totally different fonts don’t trigger a distinct variety of traces. This may cut back the influence of the shifts, nonetheless, for a lot of use-cases (e.g. generic paragraphs) will probably be tough to generalize a minimal peak.

What I’m most enthusiastic about to resolve this difficulty, 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 selection due out very quickly might lastly give us a better 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 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 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 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 peak initially earlier than the preliminary template is even inserted.

<div id="app" type="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 consumer. By default, an empty div has a peak of 0px, so give it a min-height that’s nearer to what the precise peak will probably be when the app masses.

Guarantee Person Interactions Full Inside 500ms

Person 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 advanced 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 through the use of 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 hold of it!) Efficiency tab after which click on on the file button within the high left (circled on the picture under) 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 result of latest enter. (Large preview)

You will notice 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 may nearly make out the feedback loading and the crimson footer being shifted down offscreen. Additional down the Efficiency tab, below the Expertise line, Chrome will put a reddish-pinkish field for every shift and while you click on on that you’re going to get extra element within the Abstract tab under.

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

Guaranteeing 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 consumer might even see that the enter had an impact (e.g. a loading spinner is proven) so FID is sweet, 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 probably be completed inside 500ms, however you are able to do some issues to put aside the required area utilizing the strategies above whereas that processing is occurring in order that if it does take greater than the magic 500 ms, then you definitely’ve already dealt with the shift and so won’t be penalized for it. That is particularly helpful when fetching content material from the community which might be variable and outdoors your management.

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

Synchronous JavaScript

The ultimate approach I’m going to debate is slightly controversial because it goes towards well-known net 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 under 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 under this will probably be rendered (together with the second type block to unhide it) and so no shifts will probably be incurred.

.cls-inducing-div {
    show: none;

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

.cls-inducing-div {
    show: block;

It is very important 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 in regards to the above approach is that it nonetheless unhides the content material even when the JavaScript fails or is turned off by the browser.

This system can even even be utilized with exterior JavaScript, however it will 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 positive will make some net efficiency individuals 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 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 might be accomplished server-side, this was a static website with no potential 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, fairly than shifting the content material down. We most well-liked 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 website house owners might desire to be on the high of the web page for numerous causes.

I additionally used this system on one other web page the place JavaScript strikes content material round into “foremost” and “apart” columns (for causes I received’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, prevented the CLS points that had 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 cause and the unshifted content material is proven.

Utilizing this system can influence different metrics (significantly LCP and likewise First Contentful Paint) as you might be delaying rendering, and likewise probably blocking browsers’ look forward preloader, however it’s one other instrument 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 submit, we’ve mentioned numerous ideas and tips to keep away from this. I’m glad the highlight the Core Internet Vitals have shone on this irritating difficulty — for too lengthy we net builders (and I undoubtedly embody 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 probably be helpful while you do. Who is aware of, you could even handle to get right down to the elusive 0 CLS score for all of your pages!

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