[aliem]

sice CSS3 is still a work in progress browsers implement upcomig features using -webkit/-o/-moz there's nothing wrong with it (as long than you don't use the non-valid ie's filter crap)

If you take a look here: http://www.w3.org/Style/CSS/current-work you can clearly see that pretty much everything is in Working Draft state so isn't really stable

[jarek]

It may be valid according to the wording, but denying supported features to all but one rendering engine is certainly not compatible with the spirit of any web standard.