Comment 7 for bug 1402975

Yes, "left: 50%" on a relative element will place it in the middle of the containing block. But in this case, the containing block is the document element and the width of the document element is the width of the visible viewport minus the width of the vertical scrollbar if there is one (in this case, 341px). Testing this in responsive design mode in Firefox and Chrome shows the same layout issue as Oxide, as expected - note, don't turn on mobile emulation - just set the size manually. More on this.....

The reason this works in other mobile browsers is because they set a minimum viewport width (in Chrome for Android's case, it's 980px). We should probably do the same in Oxide on mobile form factors to fix this.

(That doesn't change the fact that setting a margin in pixels will scale poorly on different form factors)