Question

I’m trying to find out the difference between using one colon “:” and two “::” I see both of them are working with pseudo class. I have tried this with before pseudo class for example and when I did both ways it worked fine.

Is there any difference between two?

Answer 1

Initially there was no difference! But this notation was introduced in CSS3 to distinguish between pseudo-elements and pseudo-classes:

This :: notation is introduced by the current document in order to
establish a discrimination between pseudo-classes and pseudo-elements.
For compatibility with existing style sheets, user agents must also
accept the previous one-colon notation for pseudo-elements introduced
in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and
:after). This compatibility is not allowed for the new pseudo-elements
introduced in CSS level 3.

the according section in the spec