FYI:
pseudo-elements, and pseudo-classes, have been around a lot longer than ePub3. as a matter of fact that is one of the differences that may confuse some people...
when to use a single colon vs a double colon (:first-line or ::first-line). The double colon was introduced in CSS3 as an attempt to differentiate between a pseudo-element and a pseudo-class. Before that (CSS1 and CSS2) a single colon was used for both. It is actually recommended to use a single colon for everything
IF you are worried about backwards compatibility.
Whether a device, or software, supports pseudo elements is completely on them. Your older devices, and older ADE, don't support much of anything...but.... as I wrote... "I don't care". Does the current version of ADE (finally) fully support CSS1 (not to mention CSS2 or CSS3)??
Please make sure you
read someone's post before you start arguing against it... it tends to make for bad optics.