I'm very fond of CSS Standards and the W3C, mainly because Vormplus always pushes his (and my) pixels to the limit of strictness, so while I was writing code for a recent project I noticed some hovering issues while trying to get everything Xhtml Strict...
I asked Peter Nederlof the following question;
... could be I'm far behind recent posts etc but I was validating my .css and they tell me following; "Property behavior doesn't exist : url("csshover.htc")"
So actually using this file to omitt IE6 bugs isn't really 'valid' ?
So if somebody out there cares along, the experts advice is:
You're right, the behavior property is not a valid css property. The css spec however does allow custom properties (which do not validate either for that matter), but those should be preceded by a dash and some abbreviation, like the way firefox uses -moz-someproperty. Sadly the IE team does not prefix their custom properties, so if you really want your css to validate, you should use a conditional comment block in your html to include a separate ie.css with all the proprietary stuff IE needs.
There's a page on msdn on conditional comments with more info: Microsoft