Good job! But it reminds me of the dreaded VGUI.dll. (0_0) For those who wrote code for the GS engine it's probaby one of the worst experiences.
The documentation was almost non-existent though.
What mod(s) did you work on?
I personally had to rewrite most of mod's VGUI code to make it less leaky and more usable, and not once have I thought of using CSS instead of those INI-style theme configs. But CSS is too complicated to parse (and no one will use that anyway). :(
I believe they switched to Chromium while preparing for the OS X port. You'd think this would be good for security, but they then proceeded to update to the latest version approximately once a decade.
Also, to be fair: it was VGUI2, which, AFAIK, was introduced with Steam UI and still used in HL2. And was never made available to modders of GS.