Downgrade IE8's support of CSS Generated Content

IE8 only supports the CSS 2.1 syntax (`:before` and `:after`). It does not support the CSS 3 syntax (`::before` and `::after`). Other browsers support both for backwards compatibility. The CSS 2.1 syntax is restricted only to pseudo-elements that existed in the 2.1 spec. Future (including all CSS3-introduced) pseudo-elements are restricted to the double-colon syntax exclusively.

The syntax change was introduced to clearly distinguish *pseudo-elements* (double-colon) from *pseudo-classes* (single-colon).
Merge pull request #209 from jasonkarns/patch-1

Downgrade IE8's support of CSS Generated Content