Negative value in <shape-radius> should be invalid

[upsuper]

It doesn't make sense to have negative radius for shapes, and both Gecko and Blink agree that negative value here should be invalid.

[astearns]

Both circle() and ellipse() state that negative values for radii are invalid.

[upsuper]

Ah, okay. I guess this is not an issue then. Probably worth moving that statement to <shape-radius>, though, since negative radius doesn't sound like something useful.