Use double-colon syntax for pseudo-elements

The fact that IE8 only recognizes :before and :after with a single colon is mentioned in the notes already.

http://www.w3.org/TR/css3-content/#pseudo-elements: “Authors are encouraged to use the new two-colon forms.”
Merge pull request #692 from GeertDD/patch-1

Use double-colon syntax for pseudo-elements