feat(css/parser): Support @starting-style

crates/swc_atoms/words.txt

@@ -2213,6 +2213,7 @@ src
 srcdoc
 srcset
 start
+starting-style
 startOffset
 startoffset
 static

crates/swc_css_parser/src/parser/at_rules/mod.rs

@@ -413,6 +413,17 @@ where
 
                 None
             }
+            js_word!("starting-style") => {
+                self.input.skip_ws();
+
+                if !is!(self, EOF) {
+                    let span = self.input.cur_span();
+
+                    return Err(Error::new(span, ErrorKind::Expected("'{' token")));
+                }
+
+                None
+            }
             _ => {
                 return Err(Error::new(Default::default(), ErrorKind::Ignore));
             }
@@ -738,6 +749,13 @@ where
 
                 declaration_list
             }
+            js_word!("starting-style") => {
+                let rule_list = self.parse_as::<Vec<Rule>>()?;
+                let rule_list: Vec<ComponentValue> =
+                    rule_list.into_iter().map(ComponentValue::from).collect();
+
+                rule_list
+            }
             _ => {
                 return Err(Error::new(Default::default(), ErrorKind::Ignore));
             }

crates/swc_css_parser/tests/fixture/at-rule/starting-style/input.css

@@ -0,0 +1,10 @@
+@starting-style {
+    h1 {
+        background-color: transparent;
+    }
+    @layer foo {
+        div {
+            height: 100px;
+        }
+    }
+}