WorldWideWeb, NeXTSTEP, 1990. The browser that was also an editor.
Not a form. Not a CMS. The page itself. WorldWideWeb, the first browser Tim Berners-Lee wrote in 1990, was also an editor. Reading and writing were the same act, and every page you visited was a page you could change.
Now try it in the browser you are actually using. Click into this paragraph and type. Nothing happens.
Mosaic arrived in 1993, ran on everything, and shipped read-only. It won. The write half of the web never came back, and getting it back now means renting it: a CMS, a platform, an account, someone else's terms.
The problem it solved: publishing needed no permission, because the browser was the editor.
Source. The document text is the real thing, fetched from CERN's restoration of the original site. Links that led somewhere now gone are marked and inert, which is itself accurate to browsing in 1990. The edit behaviour is a recreation, not an emulation.