There is a fan project that runs it in a browser. There is also an official PC release that simply works.
Halo CE was built for the original Xbox and later ported to PC and Mac, and that PC lineage is why browser ports are technically possible at all. Projects have used the open-source tooling around the original engine to get it running in a tab.
Microsoft owns Halo. The fan versions are unofficial, unlicensed, and depend on files from a copy of the game you are expected to already own.
The official route is cheaper than the hassle. Halo: The Master Chief Collection includes Combat Evolved and runs on any modern PC.