I would buy your argument if we were talking about a gradient. But this isn't a gradient, it's an anti-aliased line. The CSS describes a step transition from #

I would buy your argument if we were talking about a gradient. But this isn't a gradient, it's an anti-aliased line. The CSS describes a step transition from #eee to transparent, but because it's on a diagonal, the browser is anti-aliasing the line. Note that the reason it looks right in Chrome is because Chrome doesn't anti-alias the line.