They've also got an updated (reverse engineered?) HLDS DLL too. reHLDS I believe is what it's called. Been meaning to check what benefits/cons it brings for my own server.
There are definitely some bug fixes (I've forgotten the main ones).
Could I use this to put HL1 and its mods in the browser for a LAN party?
Probably the mods will need to be reverse-engineered and reimplemented - I haven't looked at how all of this weeks, but there's a reverse-engineered Half-Life engine so I imagine that there is a reverse-engineered Counter-Strike client.dll and mp.dll there as well (compiled down to Javascript or WASM or however it's running).