css - What the difference between two double point and one in pseudo-selector -

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i bit not understand difference between :before , ::before example. 1 should use?

this differentiate between pseudo-classes (such :hover, :focus, :active) , pseudo-elements (such ::before, ::after, ::first-line).

this introduced part of css3, world wide web consortium (the w3c), because pseudo-elements introduced prior syntactic differentiation browsers support both ::before , :before.

according reference @ mdn (mozilla developer network):

browser            |  lowest version    |    support of -------------------+--------------------+------------------ internet explorer  |  8.0               |   :pseudo-element                    +--------------------+------------------                    | 9.0                |   :pseudo-element                    |                    |   ::pseudo-element -------------------+--------------------+------------------ firefox (gecko)    | 1.0 (1.0)          |  :pseudo-element                    +--------------------+------------------                    | 1.0 (1.5)          |  :pseudo-element                    |                    |  ::pseudo-element -------------------+--------------------+------------------ opera              | 4.0                |  :pseudo-element                    +--------------------+------------------                    | 7.0                |  :pseudo-element                    |                    |  ::pseudo-element -------------------+--------------------+------------------ safari (webkit)    | 1.0 (85)           |  :pseudo-element                    |                    |  ::pseudo-element -------------------+--------------------+------------------

note, css selectors recommendation:

this :: notation introduced current document in order establish discrimination between pseudo-classes , pseudo-elements.
compatibility existing style sheets, user agents must accept previous one-colon notation pseudo-elements introduced in css levels 1 , 2 (namely, :first-line, :first-letter, :before , :after).
compatibility not allowed new pseudo-elements introduced in specification.

references:


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