Parse (but do not remove) placeholder selectors

src/lib.rs

@@ -320,6 +320,7 @@ impl<'a> StyleSheetParser<'a> {
                 | TokenKind::Symbol(Symbol::Hash)
                 | TokenKind::Symbol(Symbol::Colon)
                 | TokenKind::Symbol(Symbol::Mul)
+                | TokenKind::Symbol(Symbol::Percent)
                 | TokenKind::Symbol(Symbol::Period) => rules
                     .extend(self.eat_rules(&Selector(Vec::new()), &mut self.global_scope.clone())),
                 TokenKind::Whitespace(_) => {

src/selector.rs

@@ -89,6 +89,8 @@ pub(crate) enum SelectorKind {
     Super,
     /// Super selector in an interpolated context: `a #{&}`
     InterpolatedSuper,
+    /// Placeholder selector: `%alert`
+    Placeholder,
     /// Used to signify no selector (when there is no super_selector of a rule)
     None,
     Whitespace,
@@ -129,6 +131,7 @@ impl Display for SelectorKind {
             SelectorKind::Super | SelectorKind::None | SelectorKind::InterpolatedSuper => {
                 write!(f, "")
             }
+            SelectorKind::Placeholder => write!(f, "%"),
         }
     }
 }
@@ -269,6 +272,7 @@ impl<'a> SelectorParser<'a> {
                 TokenKind::Symbol(Symbol::Plus) => self.selectors.push(SelectorKind::Following),
                 TokenKind::Symbol(Symbol::Tilde) => self.selectors.push(SelectorKind::Preceding),
                 TokenKind::Symbol(Symbol::Mul) => self.selectors.push(SelectorKind::Universal),
+                TokenKind::Symbol(Symbol::Percent) => self.selectors.push(SelectorKind::Placeholder),
                 TokenKind::Symbol(Symbol::BitAnd) => self.selectors.push(if self.is_interpolated {
                     SelectorKind::InterpolatedSuper
                 } else {

src/value.rs

@@ -304,7 +304,7 @@ impl Value {
                 }
                 Some(Value::Ident(s, QuoteKind::Single))
             }
-            TokenKind::Variable(ref v) => Some(scope.vars.get(v).unwrap().clone()),
+            TokenKind::Variable(ref v) => Some(scope.vars.get(v).expect("expected variable").clone()),
             TokenKind::Interpolation => {
                 let mut s = eat_interpolation(toks, scope)
                     .iter()