If your (X)HTML is valid and your ARIA is valid, your document is valid, so don’t worry about it. No harm, no foul.
Jared Smith, from WebAim, posted his excellent slides on the accessibility of Rich Internet Applications presented at Accessing Higher Ground Conference. In them he maintains that if your (X)HTML is valid and your ARIA is valid then you need not worry about passing validation tools which currently have varying or no support.
This poses problems for developers who want to validate their code whilst making pages as accessible as possible. As Mathew Smith (@smithytech) commented:
@smiffy problem is that the validators aren’t wired for ARIA, but removing ARIA for validation is worse. Practical results vs. purity = win!
The Mighty Steve Faulkner has already gone into detail about this in how can I validate (X)HTML and ARIA so I wont repeat what’s been expertly covered but I’m curious to know what this means to you: working on websites in the real world how does this impact your design decisions and code choices? Does validation go out the window or does pragmatism prevail?
(Twitter is lovely but useless at tracking threads – if you have comments why not be old skool and leave a comment here).