Seen with the top-level file of the devmanual:
$ tidy -q --drop-empty-elements no index.html >/dev/null
line 74 column 38 - Warning: <a> attribute "href" lacks value
The line in question is:
<h1 class="first-header">Master Index<a class="permalink" href=""><span class="fa fa-link"></span></a>
However, an empty href attribute is perfectly valid. RFC 3986 <https://www.rfc-editor.org/rfc/rfc3986.html#section-4.4> says:
"The most frequent examples of same-document references are relative references that are empty or include only the number sign ("#") separator followed by a fragment identifier."
Hey Ulrich! Thanks for the bug report. Did you talk to upstream about this weird warning? It looks like a bug with tidy. You will most likely have better luck with the upstream authors than us! ;)