[growse]

It's just a network hint. Browsers are free to ignore (and I think Firefox has a toggle to ignore it).

[LinuxBender]

and I think Firefox has a toggle to ignore it

They do. It used to be one had to modify modes in about:config but now there is a GUI for it in the settings.

[1] - https://support.mozilla.org/en-US/kb/dns-over-https