Lift restriction on buttons with implicit type=submit and command/commandfor being no-op

[keithamus]

What is the issue with the HTML Standard?

With #9841 it is possible to author a commandfor button inside a form, with the expectation that it will trigger the command. However based on the discussions in #9625 (comment) we opted to make a <form><button commandfor=>.. a no-op, instead encouraging users to add type=button to make it work. This is to prevent compatibility issues with old browsers which will end up submitting the form as they don't recogniser the commandfor= attribute.

This issue tracks an eventual goal of lifting this restriction, such that <form><button commandfor=>.. works like a invoker command button, rather than a no-op. We can lift this restriction when there is sufficiently low compat risk, which we estimate may be in ~5 years time.

This relates to #10462.