[at-fates-hands]

I'm puzzled to try and figure out when and where you would need to do something like this. OP notes, "This is important, because I don't want to change all the positions in my CSS whenever I change fonts."

Which leads me to ask how frequently and where he's changing his fonts. My design specs are usually pretty simple and consistent and would never require something like this.

[amelius]

Well, sometimes the UX designer wants to change all the fonts in the interface at once (perhaps to a slightly different font). It is cumbersome to have to reposition all the texts in those cases, because many fonts have their baselines defined in a different way.

At other times, we may want the user to define/select the font, and keep the baselines at the same position.