designMode

One assignment makes the whole page editable, headings, paragraphs and all. Microsoft built it for webmail and it refused to die.

In 2026: Still works, everywhere, perfectly. designMode and its per-element sibling contentEditable are the engine under every rich text box on the web, so browsers maintain them with real care. The rare entry here that grew up respectable. Type in the demo, it will let you.

Where it came from: Microsoft Internet Explorer 5.5 designMode, 2000. Standardised into HTML5 alongside contentEditable. MDN ↗

<h3>This page is now editable.</h3>
<p>Click anywhere and start typing. Rearrange this paragraph. Delete
   the heading. The page will not stop you, because the page is now
   a word processor.</p>
<p><i>(Nothing is saved. Restart the demo and it all comes back.)</i></p>

<script language="JavaScript">
// One line turns the entire document into a text editor.
document.designMode = "on";
</script>