Steps to reproduce

1. Take any form generated through rails containing hidden inputs (e.g. the _method or token inputs)
2. Copy the HTML source of that page
3. Paste this to: https://validator.w3.org/nu/#textarea
4. See invalid HTML

Expected behavior

We would expect the HTML produced by the Rails form helpers to be valid HTML.

Actual behavior

The HTML produced by Rails form helpers is not valid.

System configuration

Rails version: Latest (main branch)

Ruby version: Any compatible version (for the relevant project we are using 3.1.1 if it matters)

Additional context

This functionality originates from #43280 (related issue #42610).

Related discussion:
https://discuss.rubyonrails.org/t/form-with-first-field-value-is-overriden-with-a-token-like-string/74861/4

Related Firefox bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=520561