Relying on Javascript to apply CSS kind of sucks.

It would also suck to maintain RGBA definitions and HEX definitions in one stylesheet:

  body.no-rgba {
    background-color: #FFFFC0;
  }

  body.rgba {
    background-color: rgba(255, 255, 192, 255);
  }

Correct me if I'm wrong but as far as I know, you can use all of the CSS3 additions (even things like font-faces and multiple backgrounds) and browsers will ignore them if the don't support them.

I can see the HTML5 support for old browsers being useful though.