This is actually wild. I have no idea how this works. Does it somehow emulate the rendering engine of each of these games to render the map? The water in Half-Life 2: Lost Coast is just how I remember it. Very cool.
If you're lucky you might hear from the author - his handle is Jasper_ here on HN.
It's open source: https://github.com/magcius/noclip.website
I wouldn't say it emulates so much as implements a renderer for each game. It's totally nuts.