What does <plaintext> do?
The <plaintext> element was used to render HTML code as plain text. Since everything after the opening tag was rendered as plain text, there was no closing <plaintext> tag. This element is obsolete and should not be used. Instead, use the <code> or <pre> elements.

<plaintext> was intended as a way for browsers to render text without rendering the HTML. It was never properly implemented, and is now considered obsolete.