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Created attachment 1107 [details] Screenshot of the error message Validating this page: https://www.wrapp.com/ The validator complains about content attributes in meta elements: Line 5, Column 42: Attribute content not allowed on element meta at this point. I would have expected a complaint about the property element, but content is definitely allowed.
http://www.whatwg.org/specs/web-apps/current-work/#meta "If either name, http-equiv, or itemprop is specified, then the content attribute must also be specified. Otherwise, it must be omitted." The mentioned line 5 of https://www.wrapp.com/ is does not have name, http-equiv, or itemprop.
Reopening because by default the HTML5 validator also is intended to support RDFa Lite 1.1, and that allows <meta content="Wrapp" property="og:title">, etc. (e.g., as used by the Facebook Open Graph). So it seems I have a bug in the HTML5 validator that I need to fix.
Fixed in the source now, and re-deployed for http://validator.w3.org/nu and http://validator.w3.org