Valve used a very ubscure/niche HTML render engine initially for Steam (2006). The company/website behind that isn't online anymore. An older version of the Wiki had some brief info, but all these info vanished.
It talks about the Steam overlay. Parts of the Steam application were always HTML. First the little known HTML renderer from a defunct company, than Trident and later CEF.
Here's a fun one. Start Steam with `-dev` and hit F7. Widget factory VGUI edition!
Oh also, https://developer.valvesoftware.com/wiki/VGUI_Documentation