diff --git a/servo/components/style/queries/feature.rs b/servo/components/style/queries/feature.rs
index beb8656bc51044343c8ce42892d03178edd2dff1..d886bdecb37761badc534dc31e29cee43549c352 100644
--- a/servo/components/style/queries/feature.rs
+++ b/servo/components/style/queries/feature.rs
@@ -7,6 +7,7 @@
 use crate::derives::*;
 use crate::parser::ParserContext;
 use crate::values::computed::{self, CSSPixelLength, Ratio, Resolution};
+use crate::values::AtomString;
 use crate::Atom;
 use cssparser::Parser;
 use selectors::kleene_value::KleeneValue;
@@ -45,6 +46,7 @@ pub enum Evaluator {
     OptionalNumberRatio(QueryFeatureGetter<Option<Ratio>>),
     /// A resolution.
     Resolution(QueryFeatureGetter<Resolution>),
+    String(fn(&computed::Context, Option<&AtomString>) -> KleeneValue),
     /// A keyword value.
     Enumerated {
         /// The parser to get a discriminant given a string.
