scale: font-size to % of client window?

Is it possible to scale font-size to % of client window size?

as for instance : img {width: 15%} does for images

This might ensure that a defined text remains visible, for instance a 
virtual keyboard.
(can users over-ride this?)

If this is not possible, has it been considered?

thanks

jonathan chetwynd
http://www.peepo.com