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The Validator receives <dt> or <dd> tags within <figure> tags and balks, pointing users to the exact definition which states that this is valid.
There's not much information to work from in this bug report, but I guess this is about HTML 5? Is the issue reproducible with http://validator.nu/? If yes, see http://about.validator.nu/#reporting-bugs (the W3C validator just echoes what the validator.nu engine tells it). If not, please reopen along with a URL to a document with which the issue can be reproduced, or full document source if the doc is a short one.
Ah... It was not clear that validator.nu is the backend for the W3C validator. Their bugzilla has an entry saying that the specification for figure in xhtml5 just changed, and the exact same problem is now in the TODO list. Thank you for clarifying, but can you please update the documentation? I was looking all over to help in contributing code, but couldn't find any resource to help.
I've added a note about HTML 5 related bug reports, see http://qa-dev.w3.org/wmvs/HEAD/feedback.html#bugreport The code contributing documentation is available at http://validator.w3.org/contribute.html (there is a "Contribute" link at the bottom of validator pages). Please feel free to suggest improvements if you have further ones by opening a new bug report.