Auto merge of #17381 - cbrewster:custom_element_creation, r=jdm

Custom element creation

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This implements the CE-related steps when creating elements. `is` is now support by `document.createElement` and is stored on `Element`s. Only synchronously created autonomous elements are supported as async element creation and customized built-in elements both require custom element upgrade reactions.

Spec: https://dom.spec.whatwg.org/#concept-create-element

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- [X] These changes fix #17191 (github issue number if applicable).

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components/script/dom/htmlcollection.rs

@@ -152,7 +152,7 @@ impl HTMLCollection {
     }
 
     fn match_element(elem: &Element, qualified_name: &LocalName) -> bool {
-        match elem.prefix() {
+        match elem.prefix().as_ref() {
             None => elem.local_name() == qualified_name,
             Some(prefix) => qualified_name.starts_with(&**prefix) &&
                 qualified_name.find(":") == Some(prefix.len()) &&