I just thought I'd rave about this for a moment:
http://twitter.github.com/bootstrap/

I don't normally like grid concept systems, but this one is fantastic. I'm using it for my games client, and it's solved a lot of problems in beautiful ways in a cross browser fashion for me. Also I'm using this for scalable vector based fonts:

http://fortawesome.github.com/Font-Aweso...

If you're thinking about making a web client, or really anything in html/css/js, I highly recommend bootstrap.

I second this. I'm using Bootstrap for my Ruby / Rails Majormud remake at http://apathydrive.com

Bootstrap has definitely made it easier to gloss over the design portion for now and focus on the game mechanics while I'm early in development.