Add subpixel-rendering mixin

This subpixel-rendering hack is from MAXIMILIAN HOFFMANN on his blog.
http://maximilianhoffmann.com/article/better-font-rendering-on-osx/

After the long-wait, Mozilla finally decided to add a CSS property to hangle font rendering on OS X.
That's great, because I'm now feeling texts are rendered too bold on dark backgrounds, thanks for the
subpixel-rendering.
Adjust menu font rendering
Add pre-defined font list for Chinese
Make features in feature list self-clear
Fix video height
Adjust feature list icon position
set menu breakpoint as variable in JS
Add <main> tag
Update TODO