Check out example codes for "difference between :after and ::after". It will help you in understanding the concepts better.
Code Example 1
Pseudo-elements were denoted with a single colon in CSS2, but have been changed in CSS3 "in order to establish a discrimination between pseudo-classes and pseudo-elements". For compatibility reasons a single colon is still allowed for the pseudo-elements defined in CSS 1 and CSS2. CSS2 The ':before' and ':after' pseudo-elements can be used to insert generated content before or after an element's content. CSS3 Pseudo-elements A pseudo-element is made of two colons (::) followed by the name of the pseudo-element. 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 this specification.
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