Which is absurd, because antialiasing exists for that very reason. I should be able to render a single-pixel-thick vertical line between two columns of display pixels, and get two columns of display pixels at 50% (apparent) brightness. Subpixel rendering would be even better.
Gecko has supported fractional pixel values for years in general, though for borders in particular the width is clamped to integer _device_ (not CSS) pixels.
WebKit has been rounding them at parse time (even in cases when 1 CSS px is multiple device pixels) for a while, but they're about to fix that.
I believe that IE also supports subpixel layout. Not sure about Opera, offhand.
