Auto merge of #23192 - jackxbritton:issue-23134, r=jdm

Implement DocumentFragment XML serialization.

When serializing a DocumentFragment node, we should follow [this guy](https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-a-documentfragment-node) and serialize the node's immediate children. This commit makes that change.

---
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- [X] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [X] These changes fix #23134 (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___ well, they probably do, but I don't know how to do that and could use some help.

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components/script/dom/servoparser/html.rs

@@ -10,6 +10,7 @@ use crate::dom::bindings::root::{Dom, DomRoot};
 use crate::dom::bindings::trace::JSTraceable;
 use crate::dom::characterdata::CharacterData;
 use crate::dom::document::Document;
+use crate::dom::documentfragment::DocumentFragment;
 use crate::dom::documenttype::DocumentType;
 use crate::dom::element::Element;
 use crate::dom::htmlscriptelement::HTMLScriptElement;
@@ -166,7 +167,7 @@ fn rev_children_iter(n: &Node) -> impl Iterator<Item = DomRoot<Node>> {
 impl SerializationIterator {
     fn new(node: &Node, skip_first: bool) -> SerializationIterator {
         let mut ret = SerializationIterator { stack: vec![] };
-        if skip_first {
+        if skip_first || node.is::<DocumentFragment>() {
             for c in rev_children_iter(node) {
                 ret.push_node(&*c);
             }

tests/wpt/metadata/MANIFEST.json

@@ -642622,7 +642622,7 @@
    "support"
   ],
   "domparsing/XMLSerializer-serializeToString.html": [
-   "cb8ed9372df5e05588cbd593c40a8c13ee868bec",
+   "23b9ce841f5046f180ce78d92d7ac1132712f999",
    "testharness"
   ],
   "domparsing/createContextualFragment.html": [

tests/wpt/web-platform-tests/domparsing/XMLSerializer-serializeToString.html

@@ -204,6 +204,14 @@ <h1>domparsing_XMLSerializer_serializeToString</h1>
   assert_equals(serialize(root2), '<root xmlns:xl="http://www.w3.org/1999/xlink" xl:type="v"/>');
 }, 'Check if no special handling for XLink namespace unlike HTML serializer.');
 
+test(function() {
+  var root = new DocumentFragment;
+  root.append(document.createElement('div'));
+  root.append(document.createElement('span'));
+  assert_equals(serialize(root), '<div xmlns="http://www.w3.org/1999/xhtml"></div><span xmlns="http://www.w3.org/1999/xhtml"></span>');
+}, 'Check if document fragment serializes.');
+
+
 </script>
  </body>
 </html>