Pseudo elements should be reference with a double colon, not the legacy single colon

[cookiecrook]

CSS Generated Content (a.k.a. pseudo elements) should be referenced with a double colon (::before and ::after), not the legacy single colon.

Using a single colon for pseudo elements (:before and :after) is a legacy syntax from CSS 1 or 2. Single colons are still valid on pseudo classes (states of real DOM elements, such as :visited), but a single colon is not the recommended syntax for generated elements.