Set font size in px on html tag

The trick used here was designed to honor the font setting in a user's
browser by assuming a typical default of 16px and computing the base
font size in ems relative to that.

The problem is that even if everything is sized in ems there are
sometimes aggressive user agent styles that complicate things by
restoring the system font. It turns out it fixes the various things
I've noticed in my FF this way.

h/static/styles/common.scss

@@ -42,8 +42,8 @@ p {
 em { font-style: italic; }
 
 html {
-  font-size: $base-font-size / 16px * 1em;
-  line-height: $base-line-height / 16px * 1em;
+  font-size: $base-font-size;
+  line-height: $base-line-height;
 
   @include yui_grid();