I'm using border radius styles to make a tab effect by setting the radius on the top-left and top-right borders.

Unfortunately this doesn't seem to be supported by this htc file.

Would this be possible as an addition?

Thanks.

Not possible using this method.

Confirmed. Not possible using VML ( this method.) Try more on SVG

One option might be use four different V:Curve instead of a single RoundRect as is currently done? That might be a bit hackish.... but it seems there is no other way to fully implement border-radius http://www.w3.org/TR/css3-background/#the-border-radius