I’m glad you asked! This is a beautiful property because it does one thing and one thing only killin’ natzis, wait no, it creates a stacking context!
Stacking context’s are kinda complicated if you haven’t come across them before, but one way of looking them is as a tool to group the stacking of all child elements into a single stack. So if something is on top of this stack (for example if another element is placed later in the DOM - things that come later are naturally placed on top of things that come before), then a stacking context guarantees that no item within the stack can be on top of anything that is on top of the stack.
One common reason to explicitly create a stacking context is to make sure that dialogs and tooltips in your app are rendered on top of all other content, regardless of which z indices are used. If isolation: isolate is placed on the <main> HTML element, and you render your modal after <main>, but still within <body> (perhaps by using a Portal), then it is guaranteed that the modal will appear on top of all content within <main>.
<style>
main {
isolation: isolate;
}
</style>
<body>
<main>
<p style="position: relative; z-index: 99999999">
Even though this paragraph has a crazy high z-index...
</p>
</main>
<div role="dialog">...this dialog will still render on top of the paragraph above</div>
</body>The isolation: isolate creates a “group” (stacking context) for all the children of the element it’s applied to; <main> in our example here. So usage of z-index within <main> can reorder the layering of elements within the group, but can’t affect layering outside of the group.
Intelligent use of stacking contexts prevents the need for things like z-index: 999999999, which don’t work half the time anyway because of all the places that stacking contexts are implicitly created (e.g. setting opacity to a value less than 1).