December 3rd, 1990: The earliest known HTML document: http://www.w3.org/2012/08/history-of-the-web/origins.htm

urtext.html

<h1>Standardisation</h1>
There was not a lot of discussion of this at <a href=Introduction.html>ECHT90</a>, but there seem to be two leads:
<ol>
<li><a href=People.html#newcombe>Steve newcombe's</a> and Goldfarber's "Hytime" committee
looking into SGML, and
<li>An ISO working group known as MHEG, "Multimedia/HyperText Expert Group".
led by one Francis Kretz (Thompsa SA? Rennes?).
</lo>

@seppo0010, from the article:

Note the error on the last line. If you look at documents from the period between early December and the end of the year, it is clear that SGML documents have been copied across without much change relying on the fact that illegal tags would be ignored.