Re: Only Border-radius FIX.
Thanks for the input! This has been suggested a few times before and I've actually prototyped it out a bit. Unfortunately, border-radius is not something that can be easily isolated -- rendering rounded corners requires a full re-rendering of the element's border and background, and therefore requires most of the code around parsing and rendering borders and background properties. So the file size improvement ends up not being nearly as significant as you'd expect, and that's put it pretty far down my priority list.
That said, PIE 2.0's new loading mechanism was partially designed with this sort of feature-specific build in mind, so I'm not against the idea. If someone wanted to step up and do the work to figure out what parts of the code could be eliminated, I'd welcome it.