According to https://www.mediawiki.org/wiki/Extension:TemplateStyles, @import rules should work so long as the external domain is explicitly whitelisted via `$wgTemplateStylesAllowedUrls`. However, they don't work.
To reproduce, add the following to any template style sheet and hit PREVIEW:
```@import 'https://www.example.com/styles.css';```
You get the error 'Unrecognized or unsupported rule at line 1 character 1.'
* I tried with `@import url( 'https://www.example.com/styles.css' );` and got the same result
* I verified that this isn't due to the domain not being listed in `$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