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

CSS3 changes the way pseudo-elements are selected, as the W3C wanted to distinguish pseudo-classes like a:visited from pseudo-elements like p::first-line. See Advanced CSS Selectors.