Subpixel rendering(non-retina only): Half-pixel margin causes incorrect border-bottom rendering.

• Component: Layout and Rendering
• Status: NEW ---

Created attachment 242773 [details]
Reduced test case

With some specific CSS rules, the border-bottom of an element is rendered too light or not at all. See the attached test page and screenshot.

Requirements to trigger:
* margin-top: half-pixel value (i.e., 10.5px, or 0.65625em which is 10.5px if 1em=16px)
* border: 1px solid with some color
* border-[left/right]-width: 0px
* border-bottom-[left/right]-radius: >= 2px

Two elements with these rules should be placed after each other. Depending on the elements above it, one of these will have the rendering issue:

If one of border-bottom-[left/right]-radius is set, the border-bottom is rendered only half visible; if both are set, the border-bottom is not rendered at all.

The issue might seem far-fetched, but I encountered it in a real project I'm working on. Especially when using em's for margins you can't foresee half-pixel margin values, and thus trigger this issue.

Tested in:
* Safari 7.1 (9537.85.10.17.1) with WebKit 600.1.17
* Nightly: Safari 7.1 (9537.85.10.17.1, r176637) with WebKit 601.1.11+

Created attachment 242774 [details]
Screenshot (showing bug and expected result)

[zalan]

Works fine on retina display. (1x display issue only)