HTML page has non-empty title [2779a5]: Added HTML specs reference

[giacomo-petri]

Closes issue(s): #2168

This PR adds a reference to the HTML specs, to clarify what happens if the title is not unique within a doc.

Need for Call for Review:
This will require a 1 week Call for Review

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[carlosapaduarte]

Agree this does not need to go for call for review being only an editorial change in an assumption