Would be good to make anti-aliasing optional on border-radius. For some designs it’s almost essential to have anti-aliased corners, but other times the design requires sharp ‘pixel art’ style rounded corners.
As a side-issue, it would be good to make anti-aliasing a configurable option on all elements. Being able to turn anti-aliasing off for large font sizes, for example, would sometimes be very useful.
And perhaps anti-aliasing should offer multiple levels, rather than just on/off. Photoshop offers 4 levels of anti-alias for text, and I use each one regularly to achieve the desired effect.