[FIX] compiler: fix xmlns attribute not being set correctly in firefox

Firefox will not serialize the xmlns attributes of node inside
XMLDocuments, see https://bugzilla.mozilla.org/show_bug.cgi?id=175946

To work around this issue, we replace the xmlns attribute (which is
present but won't be serialized) with a different attribute before
serializing, and then replace that attribute back with xmlns after
serialization. While this is not perfect, the risk of collision with
user content inside a template is minimal.

src/compiler/code_generator.ts

@@ -49,6 +49,12 @@ export interface CodeGenOptions extends Config {
 // of HTML (as we will parse it as xml later)
 const xmlDoc = document.implementation.createDocument(null, null, null);
 
+// The following is a workaround for firefox not serializing the xmlns attribute
+// https://bugzilla.mozilla.org/show_bug.cgi?id=175946
+const nameSpaceEl = xmlDoc.createElement("t");
+nameSpaceEl.setAttribute("xmlns", "http://www.w3.org/2000/svg");
+const canSerializeXmlns = nameSpaceEl.outerHTML.includes("xmlns")
+
 const MODS = new Set(["stop", "capture", "prevent", "self", "synthetic"]);
 
 let nextDataIds: { [key: string]: number } = {};
@@ -148,7 +154,18 @@ class BlockDescription {
     // append dom to any element and use innerHTML instead
     const t = xmlDoc.createElement("t");
     t.appendChild(this.dom!);
-    return t.innerHTML;
+    let xmlString = t.innerHTML;
+    if (canSerializeXmlns || !t.querySelector("[xmlns]")) {
+      return xmlString;
+    }
+    // The following is a workaround for firefox not serializing the xmlns attribute
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=175946
+    t.querySelectorAll("[xmlns]").forEach((node) => {
+      const nameSpace = node.getAttribute("xmlns");
+      node.removeAttribute("xmlns");
+      node.setAttribute("__firefox_bug_175946_xmlns", nameSpace!);
+    });
+    return t.innerHTML.replace(/__firefox_bug_175946_xmlns/g, "xmlns");
   }
 }