Allow `!important` qualifiers in `style[amp-noscript]`

Description

This is a follow-up to #20609.

In order to ensure that amp-accordion sections are initially-expanded on a page in which the user has JS turned off, the following is needed:

<noscript>
	<style amp-noscript>
		amp-accordion > section:not([expanded]) > :last-child {
			display: block !important;
		}
	</style>
</noscript>

Example: https://bento-amp-noscript-styles.glitch.me/ See: https://github.com/ampproject/amphtml/issues/20609#issuecomment-901426550

The !important is needed here in order to override this !important style from ampshared.css:

https://github.com/ampproject/amphtml/blob/22ff7ae97577bd1d0376d66ab0ca741cbdba4e50/css/ampshared.css#L567-L570

However, this is not currently feasible since the use of !important qualifiers is not allowed in style[amp-noscript]:

Usage of the !important CSS qualifier is not allowed.

I propose that they be allowed in the same way they are allowed in style[amp-runtime].

Interestingly, allow_important: true doesn’t appear in style[amp-runtime]:

https://github.com/ampproject/amphtml/blob/22ff7ae97577bd1d0376d66ab0ca741cbdba4e50/validator/validator-css.protoascii#L1317-L1339

Just as it doesn’t appear in style[amp-noscript]:

https://github.com/ampproject/amphtml/blob/22ff7ae97577bd1d0376d66ab0ca741cbdba4e50/validator/validator-css.protoascii#L1341-L1448

Perhaps allow_important is only considered for rules that are enabled_by: "transformed"?

Alternatives Considered

None.

Additional Context

No response

[Gregable]

I implemented this back in January, the issue was just never closed.

@jcastilloa you are probably running into issues around using !important in <style> tags outside of <noscript>, which is different and intended.

[honeybadgerdontcare]

@westonruter for <style amp-runtime ...> note that there is no cdata set in the TagSpec. Basically anything can be in the cdata for that. This is permitted as AMP Caches will rewrite the cdata with the corresponding AMP CSS with the AMP Runtime version that is being served by that AMP Cache. Also the AMP Runtime will rewrite the AMP CSS if necessary.

So allow_important would need to be set to the cdata for <style amp-noscript> TagSpec.