TYPO3 Core

IMHO you should classes instead. Allowing arbitrary font sizes completely destroys a layout.

of course you are right. but as far as we have this button in RTE it has to work right. otherwise we should completly remove this button.

I think the font-size button uses the browsers edit features directly. So it's probably Firefox and IE that create those tags.

As for removing: I never ever allow any RTE button that adds inline style to my tags.

With patch attached to issue #7970, the RTE will use span tag with style attribute in all browsers for fontstyle and fontsize. See #7970 for more details.