This (and the contenteditable attribute) were added in Internet Explorer 5.5 to let you create WYSIWYG editors and it was later reverse engineered and replicated in other browsers.<p>It is still the basis for a lot of WYSIWYG editors, but it's not perfect and sometimes provides too little control (produces messy HTML), so sometimes people avoid it and build a WYSIWYG editor from scratch using something like <canvas> (e.g. Google Docs) but this is way more effort.<p>Some of the O.G. WYSIWYG editors (e.g. CKEditor) still use "contenteditable" but patch over all the problems with it.
MDN says `document.designMode = "on"` is supported in all browsers, not just chrome dev tools. Looks like it's a rather ancient feature—in Firefox since 1.0, same with Chrome.<p><a href="https://developer.mozilla.org/en-US/docs/Web/API/Document/designMode" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/API/Document/de...</a>