Make sure you're trying to get into the game mode you downloaded when prompted with "Select game data source" - if it was HLDM, only multiplayer is going to work.
This is why browser gaming isn't taking off. User interface quirks.
Even in native you're likely to trigger Windows' "sticky keys" feature by pressing shift repeatedly (a standard keybind in many games). Back in the day, games used to crash on alt-tabbing so sticky keys killed your game until disabled.
Microsoft's xCloud only streams controller games for now, and I think that has provided an exceptional user experience early on. You can select the game in your browser, and pick up the different input device while the stream loads. Once you're done playing, put the controller down and Ctrl-W to close the tab and go back to the real-world.
The site itself seems to currently down but I was using just a few weeks ago. I found that port pretty impressive because multiplayer also works and still had lots of active players! (though ctrl-w for crouch and move forward doesn't, so I can't say it was a seamless experience)
This is what i was reminded of, but didn't remember the URL.
Sadly main reason i was reminded of it is because the Doom 3 one seems to have better responsiveness for mouselook - the HL version feels a bit too laggy.
(though even the D3 version has a tiny amount of lag which i do not remember when i tried it a couple of years ago - perhaps changes in the browser are to blame)
Works extremely well, until I try crouch-jumping (space-ctrl-W). Firefox interprets this as ctrl-w and asks me if I want to close the tab (and the mouse pointer doesn't get re-confined again).
I'm not even using a QWERTY layout, so I have to contort to press W... It's a common issue with US-made games.
That's on sway. I can try KDE, but I'd be surprised if the result is different. Just pressing ctrl-w opens the popup and definitely breaks pointer confinement.
There is also "uplink", which is Half-Life: Uplink, the demo of the game. Those are several levels that were originally part of the retail game but were cut and then used as a demo.
I thought the error produced by following the link to the original author's site was related to the .ua domain and current events, but it turns out the story is evidently more complicated, and seemingly also unresolved since 2018: https://github.com/iCrazyBlaze/Xash3D-Emscripten/issues/1
I got a little tripped up trying to do the long jump section of the hazard course, since holding control for crouching while moving forward (W) triggered the close tab shortcut. Luckily the arrow keys work for forward-backward motion.
Very smooth, surprised to see that it almost doesn't use any CPU! Basically the opposite of the WebGL stuff that I tried out in the past. I wonder why :o|
Now that made me do a double take.