Custom elements are incredibly powerful. Not only do they have their own tag name for easy selecting from css without having to use a class, but by adding custom attributes (eg size=“large”) you can basically eliminate the need for css classes entirely. Combine that with attr() to put the attribute value anywhere in or around the element with pure css and plenty of interactive components can be built purely with css, no scripting required. You can even use the @media scripting block to add noscript behavior to custom elements from css.

And then you can register a javascript class with customElements.define to add more dynamic behavior, and the sky becomes the limit. Custom elements are like a hidden framework built right into the browser.