CSS Pseudo Element
Pseudo-Element Selectors is a category of CSS Selector Syntax.
“Pseudo-element” means part of an element, such as first letter, first line.
Here is a complete list of pseudo-element:
::after→ CSS: Insert Content ::Before, ::After
::before→ CSS: Insert Content ::Before, ::After
::first-letter→ [see CSS: ::first-letter]
::first-line→ [see CSS: ::first-line]
Pseudo Element Syntax, One Colon or Two
Note: in CSS 2, pseudo-element syntax is with 1 single colon.
But in CSS 3, it's changed to 2 colons,
to distinguish it from
CSS Pseudo Class
This change is made so that these are considered CSS pseudo-elements, and has a syntax different from
[see CSS Selector Syntax]
Single colon is still supported in browsers, for backward compatibility.
As of 2018, most websites still use 1 single colon.