Question

I’m seeing this usage a lot. Specially with the pseudo classes.
What does “::” means in CSS?

.a_demo_three::before {

Answer 1

From the MDN:

Sometimes you will see double colons (::) instead of just one (:).
This is part of CSS3 and an attempt to distinguish between
pseudo-classes and pseudo-elements. Most browsers support both values.

So, when you want to use pseudo-classes like :hover, :first-child etc, use a single colon. If you want to use pseudo-elements, like ::before, ::after, ::first-letter and so on, use double colons.

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One more note: the W3C states that browsers should only accept the :: notation for pseudo-elements introduced in CSS 3, so you should follow the recommendations above 🙂