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Created attachment 1063 [details] Screenshot of the error described above. The meta tag "msapplication-window", used by Internet Explorer 9 for Pinned Websites, is marked as non-existing according to HTML elements and WHATWG wiki, even when this tag and all needed to make a website pinning-compatible are ALREADY REGISTERED in http://wiki.whatwg.org/wiki/MetaExtensions as "Registered Extensions". I am validating an HTML5 website's home page.
Fixed. https://bitbucket.org/validator/syntax/changeset/82beb5ce7a8e Thanks for the report. Please retry validating the page now.