Why your page jumps around while it loads
That lurch where you go to tap a link and the page shifts under your thumb has a specific cause, a name, and a fix that takes two attributes. Google measures it, and it is one of the three Core Web Vitals.
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What this checks
- Image dimensions setMissing dimensions cause page jump (CLS) - hurts both UX and Core Web Vitals.
Why it matters
When an image has no width and height in the HTML, the browser does not know how much room to leave for it. So it lays out the page without it, then the image arrives and shoves everything down. Google calls the resulting score Cumulative Layout Shift and treats it as a ranking signal.
The ranking side is real but it is not the main cost. The main cost is the person on a phone, on a slow connection, who taps the wrong thing because the button moved, and leaves. It is one of the most reliable ways to lose a visitor who had already decided to act.
It is also one of the few web performance problems with a genuinely simple fix. You are not rebuilding anything or buying a plugin. You are telling the browser in advance how big the picture is.
How to fix it
Put width and height on every img tag
The actual pixel dimensions of the file. Modern browsers use them to work out the aspect ratio and reserve exactly the right space, and your CSS can still resize it responsively afterwards. This single change fixes the majority of layout shift on most sites.
Reserve space for anything else that loads late
Embedded videos, maps, ad slots and injected banners cause the same jump. Give their containers a fixed height, or an aspect-ratio in CSS, so the gap exists before the content arrives.
Be careful with custom fonts
A web font swapping in for the fallback reflows text. Using font-display: swap with a fallback of similar metrics keeps the movement small.
Never insert content above what somebody is already reading
Cookie bars, promo banners and notification strips that push the page down after it has settled are the most irritating version of this, because they move the page at the exact moment attention has landed.
Questions
- Does this measure my actual page speed?
- No, and that is worth being clear about. This checks one specific and very common cause of layout shift, which is images without dimensions. A full Core Web Vitals measurement needs the page rendered in a real browser. This finds the cheap fix first.
- My images are responsive. Do width and height break that?
- No. This trips a lot of people. Set the attributes to the real file dimensions and let CSS control the displayed size, usually with width 100% and height auto. The browser uses the attributes only to work out the ratio and reserve space.
- What is a good CLS score?
- Google considers 0.1 or below to be good and anything above 0.25 to be poor. Missing image dimensions are the single most common reason a site lands in the poor band.
The other seven checks
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Layout shift is usually a symptom
The free check tells you what is broken. The audit is a dated report, tested by a person, that shows what was checked and when. That record is what protects you when somebody complains, and it is the part an automated scan cannot give you.
This is an automated check of what is publicly visible on your website. It is a plain language summary, not legal advice, and passing it does not by itself make you compliant with any particular law. Where a rule is mentioned, confirm it against the source before acting on it.