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.

It looks like you'd use this if you wanted to take advantage of newer features in some browsers today, while still maintaining separate code for browsers like IE.

If it's that simple there's no need for javascript. Just code to the latest supported standards and use conditional comments to include a secondary stylesheet for IE.