<plaintext>
It was used to indicate the end of the html part of a document. It has been replaced by </html>
The HTML 3.2 standard, suggests, probably incorrectly, that this is equivalent to <pre>.
Browsers
IE3, MOS3 interpret it as a container tag that switches off the
interpretation of HTML mark up until a </plaintext> tag
is encountered.
ARA, NN3 and OP2 interpret it as equivalent to </html> meaning
that from the tag to the end of the document, do not interpret
HTML mark up at all.