What does the double colon (::) mean in CSS?

Question

What does the double colon (::) mean in CSS?

For example:

input[type=text]::-ms-clear { display: none; }

Answer 1

It means pseudo element selector. It means the element to the right doesn't exist in the normal DOM, but can be selected.

A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.

Source

It was originally only a single colon, but was changed to differentiate it from pseudo classes (like :hover, :first-child, :not etc). It's best to use : for before and after pseudo elements since the single colon has better browser support, namely in earlier IE versions.