$ html5test

A quick check of what your browser supports: HTML5 elements and APIs, media codecs, CSS features and JavaScript language features. Every test runs locally in your browser. Features are tested by use where possible, not just by browser name.

This is a compact feature check, not a full conformance suite. "Not supported" can also mean a feature is turned off by settings, privacy mode or a non-secure context (some APIs need HTTPS, which this site uses).