font-size

font-size is always inherited by default, i.e every html element gets the font-size of the parent element... The following link from W3C site confirms this..
http://www.w3.org/TR/CSS2/propidx.html

font-size relative values are always relative to the font-size of the parent element.
http://www.w3.org/TR/CSS2/fonts.html#value-def-relative-size

If no font-size is specified then the default font size specified by the browser is considered(and this value can be changed by the end user)...

So setting the font-size to relative units should not cause any problems interms of accessiblity or cross browser compatability.