Having troubles validating

Afternoon,

Currently have a quick question or best practise (possibly a bug). Having
troubles validating . . . #c00 {width:expression(document.body.clientWidth <
780? "780px": "auto" ); float:left}

Any comments and is there some syntax I can incorporate to solve the
problem.

Currently developing www.directgov.gov.uk and www.dh.gov.uk.

Kind Regards
Donovan Justice