RFR: 8364149: Conditional stylesheet imports [v4]

[Andy Goryachev]

On Thu, 26 Feb 2026 00:49:18 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> The [`@import`](https://www.w3.org/TR/css-cascade-5/#at-import) rule is extended to support conditional stylesheet imports:
>> 
>> 
>> @import [ <url> | <string> ] <media-query-list>? ;
>> 
>> 
>> Conceptually, a conditional import takes all rules of the referenced stylesheet, literally includes them at the location of the `@import` rule, and surrounds them with a `@media` rule with the specified `<media-query-list>`.
>> 
>> The implementation of this feature extends media queries with a context-free evaluation function:
>> 
>> interface MediaQuery {
>>     TriState evaluate(); // returns TRUE / FALSE / UNKNOWN
>> }
>> 
>> 
>> This mode of evaluation uses Kleene's strong trivalent logic to determine whether the media query will always match (it's a tautology), will never match (it's a contradiction), or whether it depends on the context or we simply don't know.
>> 
>> Using this mechanism, a conditional stylesheet import that can never match will be skipped at runtime. Similarly, a conditional stylesheet import that will always match doesn't need the `<media-query-list>` at all; its rules will be pasted verbatim into the main stylesheet.
>> 
>> Note that this mechanism is _not_ meant to be a full-blown theorem solver. It won't detect that, for example, the following import conditions will always match:
>> 
>> @import url("stylesheet.css") (width > 1000) or (width <= 1000);
>> 
>> 
>> Its purpose is an optimization for "obvious" cases. For example, at the moment the built-in themes use hard-coded conditions to include user-agent stylesheets at runtime:
>> 
>>     // in PlatformImpl.java:
>>     if (isSupported(ConditionalFeature.INPUT_TOUCH)) {
>>         uaStylesheets.add("com/sun/javafx/scene/control/skin/modena/touch.css");
>>     }
>> 
>> In the future, we might have a way to test for `ConditionalFeature` flags that achieves the same thing, but with a public API available to all theme authors:
>> 
>> @import url("com/sun/javafx/scene/control/skin/modena/touch.css")
>>         (-fx-supports-conditional-feature: input-touch);
>
> Michael Strauß has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - specify capacity of MediaQueryList
>  - javadoc change

Thanks!  You are right, I do see a cryptic message

`WARNING: CSS Error parsing file:/Users/XXX/Css_Conditional_8364149_Main.css: Expected LPAREN at [1,69]`

I wish we were using more human-readable error messages, but that's another topic.

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PR Comment: https://git.openjdk.org/jfx/pull/2031#issuecomment-3967894878