Removed vendor prefixes, except where it fixes a specific bug or quirk
Inherit box-sizing for best practice

Using inheritance allows easy override on other elements, and also it’s the best practice.
Remove border-radius mixin
Closing DIV comment for site-branding

Call it OCD, but I feel like we need a closing DIV comment for  ```site-branding``` :)
_s: Merge branch 'pr/561'.
_s: Remove remaining box-sizing prefix.

Also removes the box-sizing mixin, as it is of no use anymore.

See http://caniuse.com/#feat=css3-boxsizing
See #558.

Fixes #561.
_s: Merge branch 'pr/616'.
Merge pull request #640 from emiluzelac/patch-34

Closing DIV comment for site-branding.
_s: Merge branch 'pr/609'.

Props @mor10 for documentation.
Fixes #641.
Merge branch 'master' of https://github.com/Automattic/_s