Auto merge of #11155 - emilio:codegen-dict-keyword, r=Ms2ger

codegen: Fix dictionary handling and semantics

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11152 (github issue number if applicable).

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

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

Fixes #11152

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tests/wpt/web-platform-tests/domparsing/DOMParser-parseFromString-html.html

@@ -61,4 +61,10 @@ test(function() {
     assert_equals(htmldoc.documentElement.localName, "html");
     assert_equals(htmldoc.documentElement.namespaceURI, "http://www.w3.org/1999/xhtml");
 }, "DOMParser parses HTML tag soup with no problems");
+
+test(function() {
+    assert_throws(new TypeError(), function() {
+        new DOMParser().parseFromString("", "text/foo-this-is-invalid");
+    })
+}, "DOMParser throws on an invalid enum value")
 </script>