Chrome, Firefox, and Opera set `vertical-align: -0.2em`. The browser implementations aren't great. They scale badly with font size and when the height of the element is changed. Aligning them to the baseline, as IE does, helps make their alignment consistent with other similar elements.
My hypothesis is that it's more important for a user to get the focus style they expect *within* their browser than it is to have consistent focus styles *between* browsers. In particular, replacing Chrome's default focus style (esp. just for links) seems presumptuous. Component designers/developers can make the decision on when to modify these browser defaults. Fix gh-216