Bugzilla – Bug 929
character encoding declaration is now required
Last modified: 2013-03-26 15:16:52 CET
http://html5.org/r/7160 + <p class="note">A character encoding declaration is required (either + in the <span title="Content-Type">Content-Type metadata</span> or + explicitly in the file) even if the encoding is US-ASCII, because an + encoding is needed to process non-ASCII characters entered by the + user in forms, in URLs generated by scripts, and so forth.</p> So I think the warning-level message that the parser code currently emits needs to be an error-level message instead.
https://hg.mozilla.org/projects/htmlparser/rev/4e1877dd8a41