According to https://www.mediawiki.org/wiki/Extension:TemplateStyles, @import rules should work for external domains explicitly whitelisted via $wgTemplateStylesAllowedUrls in LocalSettings.php. However, they currently do not work.
To reproduce, add the following to any template style sheet and hit PREVIEW:
You get the error 'Unrecognized or unsupported rule at line 1 character 1.'
- I tried with @import url( 'https://www.example.com/styles.css' ); and got the same result
- I verified that this isn't due to the domain not being listed in $wgTemplateStylesAllowedUrls by setting $wgTemplateStylesAllowedUrls['css'] = [ '/.*/' ]; in a development wiki
- I verified that this isn't about @ rules in general by adding @namespace 'Foo' to a template style sheet
- This issue is similar and may be related to T293633