I think this -100px top margin might be assuming a perfect 100px height on the #masthead element, but it seems to be off by a half-pixel (on a retina display). Here's what I'm seeing:

![screenshot 2014-01-25 08 48 32](https://f.cloud.github.com/assets/58062/2001644/8d0976fa-85bf-11e3-93c5-ea96733a6ea9.png)
Note the thin blue line on top of white tabzilla tab.

I can't quite track down the source of this issue.