I'm not aware of anything that considers "hidden" inputs a security mechanism, but the CSS team should probably triage and consider whether to track as a functionality bug.

I'm removing this from the security queue.

Values in <input type="hidden"> fields are not really a secret to the user, since they can be observed in devtools or even by monitoring the network traffic. The fact that they are hidden mostly reflects that the web application won't work properly if the user modifies them.

Sometimes they can be of significance to attackers who control other websites, such as is the case with CSRF tokens, but same origin policy protects it (i.e. attacker websites should not be able to affect the CSS of the target site).

As elawrence suggests, this might warrant investigation on the basis of it diverging from Firefox's behavior.

I looked at this further, and IE behaves as Chrome does.

Firefox does match the CSS value selector, as you can verify in their developer tools. The reason you don't see the network request is very likely that they've undertaken an optimization whereby they don't bother fetching the background image for a hidden input as the image is not rendered to the user.

So I believe this is working as intended.

This seems like a minor interop issue, probably very low risk of actually causing interop problems.