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In CSS3, the use of vendor prefixes is quite common. However, running such code through the CSS validator (with css3 chosen) raises an error of "property doesn't exist" for all instances. Example input is this: .box_round { -moz-border-radius: 12px; -webkit-border-radius: 12px; border-radius: 12px; } From what I can tell, this syntax is documented in the 2.1 and 3 specs: http://www.w3.org/TR/CSS2/syndata.html#vendor-keywords http://www.w3.org/TR/css3-syntax/#vendor-specific Since use of these prefixes is so widespread, the validator should cease to raise errors on their use. The validator to distinguish between an unrecognised property name, and an unrecognised property name that conforms to the syntax reserved for extensions. Marking these with an 'experimental' warning seems sufficient.
At the least vendor prefixes should generate warnings not errors, as they’re presumably being flagged as an error because they begin with a hyphen, rather than actually being validated: http://lists.w3.org/Archives/Public/www-validator-css/2009Jul/0002.html I agree with Paul’s suggestion of “experimental” and Yann’s suggestion of “inform[ing] the user that they are using a property that will be recognised only by a specific browser”. As Paul mentions there’s a big increase in vendor prefixes as we wait for implemented things to CR, so I think more people are going to have this problem.
When using the servlet version of the validator, you can choose to mark vendor extensions as warnings, as opposed to errors. Doesn't fix the issue, but is a decent workaround.