If you're someone who's found themselves knee deep in HTML, you'll know that even this so called "not a programming" language has comments. They're pretty obvious:
<!-- this is a comment in HTML -->
That's it. There's no other way to write comments in HTML. Well...
OK, so officially
<!-- comment --> is how comments should be written. But the specification will turn your HTML into a comment at the drop of a hat.
Let's have a look at the many and varied ways that parser errors should result in comments!
<!-- comment --!>
<! whatever or
<! whatever > (yes, it can even end with the EOF)
<?whatever etc. or
<![CDATA[whatever]]> (this only applies in "non-foreign" content)
<!-- comment (EOF in comment)
<script><!-- comment (EOF in script)
<666></666> (opening part is treated as text, the closing part is a comment)
On top of all those non-comment comments, a final extra quirk: the second
<!-- <!-- I wish I could nest comments --> --> is treated as mark up!