diff --git a/components/style/values/generics/grid.rs b/components/style/values/generics/grid.rs
index 56b6f463691..13c7fa54200 100644
--- a/components/style/values/generics/grid.rs
+++ b/components/style/values/generics/grid.rs
@@ -120,7 +120,9 @@ impl Parse for GridLine<specified::Integer> {
                 if val_before_span || grid_line.ident.is_some() {
                     return Err(location.new_custom_error(StyleParseErrorKind::UnspecifiedError));
                 }
-                grid_line.ident = Some(CustomIdent::from_ident(location, &name, &[])?);
+                // NOTE(emilio): `span` is consumed above, so we only need to
+                // reject `auto`.
+                grid_line.ident = Some(CustomIdent::from_ident(location, &name, &["auto"])?);
             } else {
                 break;
             }
diff --git a/components/style/values/specified/grid.rs b/components/style/values/specified/grid.rs
index ad95264c595..ec00393febd 100644
--- a/components/style/values/specified/grid.rs
+++ b/components/style/values/specified/grid.rs
@@ -95,7 +95,7 @@ pub fn parse_line_names<'i, 't>(
         while let Ok((loc, ident)) = input.try(|i| -> Result<_, CssParseError<()>> {
             Ok((i.current_source_location(), i.expect_ident_cloned()?))
         }) {
-            let ident = CustomIdent::from_ident(loc, &ident, &["span"])?;
+            let ident = CustomIdent::from_ident(loc, &ident, &["span", "auto"])?;
             values.push(ident);
         }