LibWeb/XML: Avoid placing all elements missing an ns in the HTML ns

Also remove the hack for SVG documents, a well-formed SVG document has
the correct xmlns attribute set, which should be automatically picked up
by the builder now.

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.h

@@ -42,7 +42,13 @@ private:
     XMLScriptingSupport m_scripting_support { XMLScriptingSupport::Enabled };
     bool m_has_error { false };
     StringBuilder text_builder;
-    DeprecatedFlyString m_namespace { Namespace::HTML };
+    DeprecatedFlyString m_namespace {};
+
+    struct NamespaceStackEntry {
+        DeprecatedFlyString ns;
+        size_t depth;
+    };
+    Vector<NamespaceStackEntry, 2> m_namespace_stack;
 };
 
 }