You answered your own question. It’s less known about, by the time the #rrggbbaa syntax came out people would use rgba() for alpha. Some would also argue the rg

You answered your own question. It’s less known about, by the time the #rrggbbaa syntax came out people would use rgba() for alpha. Some would also argue the rgba() syntax is more readable when alpha is involved. If alpha support came out with hex from the beginning then it may be a different story