CSS Pseudo-elements

Notice the double colon notation – ::first-line versus :first-line The double colon replaced the single-colon notation for pseudo-elements in CSS3. This was an attempt from W3C to distinguish between pseudo-classes and pseudo-elements. The single-colon syntax was used for both pseudo-classes and pseudo-elements in CSS2 and CSS1.

https://www.w3schools.com/CSS/css_pseudo_elements.asp