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HTTP endpoints that process form data and accept text/html as input

Instead of having to send application/x-www-form-urlencoded, you can just POST the markup for the page containing the original form back to the server, except with the form already filled in: all the fields have their value attributes set appropriately, i.e. whatever the submitter actually wants that field to be.

Not unlike the way real (i.e. paper) forms work. Any other prior art? (And I do mean "art"—which this is.)