That's because the standards don't list tags per se, but list generic
identifiers (aka GI, like LI, UL, IMG, etc.). When it talks about
tags, it usually talks about the start tag (like <FOO>) associated
with a GI, and not the end tag (like </FOO>).
Further, the end-tag for EMPTY elements (ala IMG) "must be omitted",
so documents using </IMG> are in error. (Ok, I don't have Goldfarb
here, but Eric van Herwijnen does agree with me).
BTW, there is a mention of </LI> in the second paragraph of 3.2.2 in
the HTML 2.0 draft (dated September 22, 1995, which has been accepted
by the IESG). Further, it is relevant to the question at hand:
Some elements only have a start-tag without an end-tag. For example,
to create a line break, use the `<BR>' tag. Additionally, the end
tags of some other elements, such as Paragraph (`</P>'), List Item
(`</LI>'), Definition Term (`</DT>'), and Definition Description
(`</DD>') elements, may be omitted.