style: Add dark mode to plaintext.css, and a document rule to target plaintext documents.

We add two @-moz-document functions: `plain-text-document()`, matching the
obvious, and `unobservable-document()`, which matches a top-level document with
no opener. This is the equivalent check we do for automatic darkening of
`about:blank` here:

    https://searchfox.org/mozilla-central/rev/014fe72eaba26dcf6082fb9bbaf208f97a38594e/layout/base/PresShell.cpp#5282

The former we don't need to use, but it's nice to let user stylesheets target
plaintext documents properly (rather than relying on extensions or what not).

Note that these are not content-observable.

Add two tests: One showing that we produce different rendering when on dark
mode, and one showing that we produce the same one from an iframe, regardless
of dark mode.

Depends on D101517

Differential Revision: https://phabricator.services.mozilla.com/D101518

components/style/stylesheets/document_rule.rs

@@ -110,6 +110,13 @@ pub enum DocumentMatchingFunction {
     /// Matching function for a media document.
     #[css(function)]
     MediaDocument(MediaDocumentKind),
+    /// Matching function for a plain-text document.
+    #[css(function)]
+    PlainTextDocument(()),
+    /// Matching function for a document that can be observed by other content
+    /// documents.
+    #[css(function)]
+    UnobservableDocument(()),
 }
 
 macro_rules! parse_quoted_or_unquoted_string {
@@ -161,6 +168,21 @@ impl DocumentMatchingFunction {
                     Ok(DocumentMatchingFunction::MediaDocument(kind))
                 })
             },
+
+            "plain-text-document" => {
+                input.parse_nested_block(|input| {
+                    input.expect_exhausted()?;
+                    Ok(DocumentMatchingFunction::PlainTextDocument(()))
+                })
+            },
+
+            "unobservable-document" => {
+                input.parse_nested_block(|input| {
+                    input.expect_exhausted()?;
+                    Ok(DocumentMatchingFunction::UnobservableDocument(()))
+                })
+            },
+
             _ => {
                 Err(location.new_custom_error(
                     StyleParseErrorKind::UnexpectedFunction(function.clone())
@@ -184,6 +206,8 @@ impl DocumentMatchingFunction {
             DocumentMatchingFunction::MediaDocument(_) => {
                 GeckoDocumentMatchingFunction::MediaDocument
             },
+            DocumentMatchingFunction::PlainTextDocument(..) => GeckoDocumentMatchingFunction::PlainTextDocument,
+            DocumentMatchingFunction::UnobservableDocument(..) => GeckoDocumentMatchingFunction::UnobservableDocument,
         };
 
         let pattern = nsCStr::from(match *self {
@@ -197,6 +221,8 @@ impl DocumentMatchingFunction {
                 MediaDocumentKind::Plugin => "plugin",
                 MediaDocumentKind::Video => "video",
             },
+            DocumentMatchingFunction::PlainTextDocument(()) |
+            DocumentMatchingFunction::UnobservableDocument(()) => "",
         });
         unsafe { Gecko_DocumentRule_UseForPresentation(device.document(), &*pattern, func) }
     }
@@ -253,10 +279,9 @@ impl DocumentCondition {
 
     #[cfg(feature = "gecko")]
     fn allowed_in(&self, context: &ParserContext) -> bool {
-        use crate::stylesheets::Origin;
         use static_prefs::pref;
 
-        if context.stylesheet_origin != Origin::Author {
+        if context.in_ua_or_chrome_sheet() {
             return true;
         }