Fix issue 033506: add a page on CLS

files/en-us/glossary/cls/index.md

@@ -0,0 +1,19 @@
+---
+title: Cumulative Layout Shift (CLS)
+slug: Glossary/CLS
+page-type: glossary-definition
+---
+
+{{GlossarySidebar}}
+
+**Cumulative Layout Shift (CLS)** is a usability metric for websites, designed by Google as one of the [Core Web Vital](https://web.dev/explore/learn-core-web-vitals) 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 {{htmlelement("img")}} or {{htmlelement("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 {{domxref("LayoutShift")}} interface, part of the [Performance API](/en-US/docs/Web/API/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](https://web.dev/articles/cls) on web.dev