Add a fast path in Element::SetInnerHTML when the value is small and trivial text

Inspired from gecko which has a similar fast path. This makes innerHTML
more than 10 times faster for this case.

Fixes #25892

components/script/dom/htmltemplateelement.rs

@@ -41,12 +41,15 @@ impl HTMLTemplateElement {
         prefix: Option<Prefix>,
         document: &Document,
     ) -> DomRoot<HTMLTemplateElement> {
-        Node::reflect_node(
+        let n = Node::reflect_node(
             Box::new(HTMLTemplateElement::new_inherited(
                 local_name, prefix, document,
             )),
             document,
-        )
+        );
+
+        n.upcast::<Node>().set_weird_parser_insertion_mode();
+        n
     }
 }