Some HTML¹ closing tags are optional, i.e.:

</HTML>
</HEAD>
</BODY>
</P>
</DT>
</DD>
</LI>
</OPTION>
</THEAD>
</TH>
</TBODY>
</TR>
</TD>
</TFOOT>
</COLGROUP>

Note: Not to be confused with closing tags that are forbidden to be included, i.e.:

</IMG>
</INPUT>
</BR>
</HR>
</FRAME>
</AREA>
</BASE>
</BASEFONT>
</COL>
</ISINDEX>
</LINK>
</META>
</PARAM>

Note: xhtml is different from HTML. xhtml is a form of xml, which requires every element have a closing tag. A closing tag can be forbidden in html, yet mandatory in xhtml.

Are the optional closing tags

• ideally included, but we'll accept them if you forgot them, or
• ideally not included, but we'll accept them if you put them in

In other words, should i include them, or should i not include them?

The HTML 4.01 spec talks about closing element tags being optional, but doesn't say if it's preferable to include them, or preferable to not include them.

On the other hand, a random article on DevGuru says:

The ending tag is optional. However, it is recommended that it be included.

The reason i ask is because you just know it's optional for compatibility reasons; and they would have made them (mandatory | forbidden) if they could have.

Put it another way: What did HTML 1, 2, 3 do with regards to these, now optional, closing tags. What does HTML 5 do? And what should i do?

Footnotes

¹HTML 4.01