Auto merge of #18888 - tigercosmos:xml, r=jdm

fix  #18776: use XML fragment serialization for innerHTML in XML documents

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I am not sure whether my solution is in the right way.
@jdm Can you give some advises?

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- [X] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #18776 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/element.rs

@@ -120,6 +120,11 @@ use style::values::{CSSFloat, Either};
 use style::values::{specified, computed};
 use stylesheet_loader::StylesheetOwner;
 use task::TaskOnce;
+use xml5ever::serialize as xmlSerialize;
+use xml5ever::serialize::SerializeOpts as XmlSerializeOpts;
+use xml5ever::serialize::TraversalScope as XmlTraversalScope;
+use xml5ever::serialize::TraversalScope::ChildrenOnly as XmlChildrenOnly;
+use xml5ever::serialize::TraversalScope::IncludeNode as XmlIncludeNode;
 
 // TODO: Update focus state when the top-level browsing context gains or loses system focus,
 // and when the element enters or leaves a browsing context container.
@@ -1004,6 +1009,19 @@ impl Element {
         }
     }
 
+    pub fn xmlSerialize(&self, traversal_scope: XmlTraversalScope) -> Fallible<DOMString> {
+        let mut writer = vec![];
+        match xmlSerialize::serialize(&mut writer,
+                        &self.upcast::<Node>(),
+                        XmlSerializeOpts {
+                            traversal_scope: traversal_scope,
+                            ..Default::default()
+                        }) {
+            Ok(()) => Ok(DOMString::from(String::from_utf8(writer).unwrap())),
+            Err(_) => panic!("Cannot serialize element"),
+        }
+    }
+
     pub fn root_element(&self) -> DomRoot<Element> {
         if self.node.is_in_doc() {
             self.upcast::<Node>()
@@ -2047,11 +2065,14 @@ impl ElementMethods for Element {
 
     /// https://w3c.github.io/DOM-Parsing/#widl-Element-innerHTML
     fn GetInnerHTML(&self) -> Fallible<DOMString> {
-        // XXX TODO: XML case
         let qname = QualName::new(self.prefix().clone(),
                                   self.namespace().clone(),
                                   self.local_name().clone());
-        self.serialize(ChildrenOnly(Some(qname)))
+        if document_from_node(self).is_html_document() {
+            return self.serialize(ChildrenOnly(Some(qname)));
+        } else {
+            return self.xmlSerialize(XmlChildrenOnly(Some(qname)));
+        }
     }
 
     /// https://w3c.github.io/DOM-Parsing/#widl-Element-innerHTML
@@ -2071,7 +2092,11 @@ impl ElementMethods for Element {
 
     // https://dvcs.w3.org/hg/innerhtml/raw-file/tip/index.html#widl-Element-outerHTML
     fn GetOuterHTML(&self) -> Fallible<DOMString> {
-        self.serialize(IncludeNode)
+        if document_from_node(self).is_html_document() {
+            return self.serialize(IncludeNode);
+        } else {
+            return self.xmlSerialize(XmlIncludeNode);
+        }
     }
 
     // https://w3c.github.io/DOM-Parsing/#dom-element-outerhtml