thoughtbot/guides — pull request #235 by penge (2014-10-10 15:12:55+00:00)
Title: Use double colons for pseudo-elements

Use double colons (::) for pseudo-elements, like ::before, ::after.
Use single colon (:) for pseudo-class, :hover, :checked.

More at:
https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements
https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes
https://developer.mozilla.org/en-US/docs/Web/CSS/::before