Question

Twitter bootstrap.css has such a code:

:-moz-placeholder {
  color: #999999;
}

:-ms-input-placeholder {
  color: #999999;
}

::-webkit-input-placeholder {
  color: #999999;
}

The question is about :: in front of -webkit-input-placeholder. Why two :: and what is that for?

Answer 1

:: denotes a pseudo-element (e.g. ::before and ::after). : denotes a pseudo-class (e.g. :link and :hover). It’s just a naming convention to differentiate between pseudo-elements and pseudo-classes. IE8 and below do NOT support the :: convention.

Here’s an explanation of these two directly from the W3C spec: http://www.w3.org/TR/CSS21/selector.html#pseudo-elements