diff --git a/components/script/dom/stylepropertymapreadonly.rs b/components/script/dom/stylepropertymapreadonly.rs
index 38632f3e6e9..02f8a342321 100644
--- a/components/script/dom/stylepropertymapreadonly.rs
+++ b/components/script/dom/stylepropertymapreadonly.rs
@@ -16,6 +16,7 @@ use servo_atoms::Atom;
 use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::iter::Iterator;
+use style::custom_properties;
 
 #[dom_struct]
 pub struct StylePropertyMapReadOnly {
@@ -73,14 +74,14 @@ impl StylePropertyMapReadOnlyMethods for StylePropertyMapReadOnly {
         // https://drafts.css-houdini.org/css-typed-om-1/#dom-stylepropertymap-getproperties
         // requires this sort order
         result.sort_by(|key1, key2| {
-            if key1.starts_with("-") {
-                if key2.starts_with("-") {
+            if let Ok(key1) = custom_properties::parse_name(key1) {
+                if let Ok(key2) = custom_properties::parse_name(key2) {
                     key1.cmp(key2)
                 } else {
                     Ordering::Greater
                 }
             } else {
-                if key2.starts_with("-") {
+                if let Ok(_) = custom_properties::parse_name(key2) {
                     Ordering::Less
                 } else {
                     key1.cmp(key2)