Cumulative Layout Shift (CLS)
Cumulative Layout Shift (CLS) is a usability metric for websites, designed by Google as one of the Core Web Vital metrics.
It measures the extent to which users encounter unexpected layout shifts, in which elements of the page are moved in an unexpected way: that is, that are not the result of a user action like pressing a button or part of an animation.
Layout shifts may be caused, for example, by <img> or <video> elements that are not given width and height attributes, so the browser doesn't know how much space they will occupy until they are loaded.
The LayoutShift interface, part of the Performance API, can be used to measure individual layout shifts, which can then be used to calculate the CLS score for a web page.
See also
- CLS on web.dev