Prior to Netscape's introduction of frames to HTML, browsers didn't understand the <frameset> and <frames> tags. It serves well to remind us that, despite their dwindling numbers, some old browsers remain that don't understand them. To ensure that all browsers load with displayable content, the <noframes> tag was introduced.

Browsers that recognise the <frameset> tag ignore the content of the <noframes> container, which remains readable by old browsers. You should use the <noframes> tag to provide a non-frames-based version of your site, or, alternatively, to harass visitors using old browsers!