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.
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.
Some of the O.G. WYSIWYG editors (e.g. CKEditor) still use "contenteditable" but patch over all the problems with it.
I love how Trix [0] and (I think) ProseMirror [1] work in that regard: it does use contenteditable, but every edit you make is applied to an internal model instead, then the editor state is updated back from the model.
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.
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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.
Some of the O.G. WYSIWYG editors (e.g. CKEditor) still use "contenteditable" but patch over all the problems with it.
[0]: https://trix-editor.org/
[1]: https://prosemirror.net/
Edit: also Quill! I think it’s pretty much the goto strategy for WYSIWYG editors nowadays: https://v1.quilljs.com/guides/comparison-with-other-rich-tex...
https://developer.mozilla.org/en-US/docs/Web/API/Document/de...