HTML and CSS Reference
4.6.9. The <plaintext> Tag (Obsolete)
Throw the <plaintext> tag out of your bag of HTML tricks; it's obsolete,
like <listing> and <xmp> , and is included here for historical reasons.
Authors once used <plaintext> to tell the browser to treat the rest of
the document's text as written, with no markup. There was no ending
tag for <plaintext> (of course, no markup!), but there was an end to
<plaintext> . Forget about it.