Auto merge of #19222 - upsuper:serialize-group-rule, r=emilio

Serialize media rule and supports rule like Gecko

This should fix [bug 1417207](https://bugzilla.mozilla.org/show_bug.cgi?id=1417207).

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components/style/stylesheets/media_rule.rs

@@ -46,12 +46,7 @@ impl ToCssWithGuard for MediaRule {
     where W: fmt::Write {
         dest.write_str("@media ")?;
         self.media_queries.read_with(guard).to_css(dest)?;
-        dest.write_str(" {")?;
-        for rule in self.rules.read_with(guard).0.iter() {
-            dest.write_str(" ")?;
-            rule.to_css(guard, dest)?;
-        }
-        dest.write_str(" }")
+        self.rules.read_with(guard).to_css_block(guard, dest)
     }
 }
 

components/style/stylesheets/rule_list.rs

@@ -7,7 +7,9 @@
 #[cfg(feature = "gecko")]
 use malloc_size_of::{MallocShallowSizeOf, MallocSizeOfOps};
 use servo_arc::{Arc, RawOffsetArc};
-use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard};
+use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked};
+use shared_lock::{SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
+use std::fmt;
 use stylesheets::{CssRule, RulesMutateError};
 use stylesheets::loader::StylesheetLoader;
 use stylesheets::rule_parser::State;
@@ -88,6 +90,21 @@ impl CssRules {
         self.0.remove(index);
         Ok(())
     }
+
+    /// Serializes this CSSRules to CSS text as a block of rules.
+    ///
+    /// This should be speced into CSSOM spec at some point. See
+    /// <https://github.com/w3c/csswg-drafts/issues/1985>
+    pub fn to_css_block<W>(&self, guard: &SharedRwLockReadGuard, dest: &mut W)
+        -> fmt::Result where W: fmt::Write
+    {
+        dest.write_str(" {")?;
+        for rule in self.0.iter() {
+            dest.write_str("\n  ")?;
+            rule.to_css(guard, dest)?;
+        }
+        dest.write_str("\n}")
+    }
 }
 
 /// A trait to implement helpers for `Arc<Locked<CssRules>>`.

components/style/stylesheets/supports_rule.rs

@@ -47,12 +47,7 @@ impl ToCssWithGuard for SupportsRule {
     where W: fmt::Write {
         dest.write_str("@supports ")?;
         self.condition.to_css(dest)?;
-        dest.write_str(" {")?;
-        for rule in self.rules.read_with(guard).0.iter() {
-            dest.write_str(" ")?;
-            rule.to_css(guard, dest)?;
-        }
-        dest.write_str(" }")
+        self.rules.read_with(guard).to_css_block(guard, dest)
     }
 }