Originally Posted by _savage
However, I do use "font-size:x-large;" (or some such) for <h2> tags, assuming that these are relative to the user-selected font size which should be "font-size:normal;".
Is that wrong? If so, how do I go properly about increasing and decreasing the sizes of fonts for headers etc relative to the user's font size?
It depends on the reader software, I guess. "normal", "x-large" and friends are supposed to be fixed sizes set by the reading software. I can imagine some software that sets "normal" to 12pt and "x-large" to 20pt... and let's you change the "normal" size, but has a different setting (or no setting at all) for the "x-large" size. That would break your usage. Or it could let you select the base size among the "normal", "x-large", etc. pre-defined sizes, but would leave your "x-large" text unchanged. That would break it too.
I think it's better so set "font-size: 150%;" (= "font-size: 1.5em;") or something like that (for <h2>), that should work unless the reader is broken already.