style: Reserve -- as a custom property name

I think this should be fairly low risk, let's try to get it done sooner
rather than later.

Differential Revision: https://phabricator.services.mozilla.com/D116456

components/style/custom_properties.rs

@@ -88,7 +88,7 @@ pub type Name = Atom;
 ///
 /// <https://drafts.csswg.org/css-variables/#typedef-custom-property-name>
 pub fn parse_name(s: &str) -> Result<&str, ()> {
-    if s.starts_with("--") {
+    if s.starts_with("--") && s.len() > 2 {
         Ok(&s[2..])
     } else {
         Err(())