I've thoroughly reviewed the entire PR. Let me summarize what I found:

1. assertions.ts changes: Adds codename_name_map, name_code_map, code_name_map lookup tables, an assert_dom helper, and assert_throws_dom_impl -- a reimplementation of the upstream WPT assert_throws_dom logic that handles legacy DOMException code names and provides better error messages. The existing assert_throws_dom is updated to delegate to the new implementation.

2. websockets-test.ts changes: Removes several expectedFailures entries for tests that were failing because the old implementation didn't handle legacy code names like SYNTAX_ERR. Also converts Close-1005.any.js from disabledTests: true to expectedFailures with a cleanup to prevent hanging.

The code is clean, correctly matches the upstream WPT testharness.js patterns, the == at line 467 is intentional (upstream uses loose equality for DOMException property comparison), and the lookup tables match the WebIDL error names table. The websocket test changes are well-motivated with clear comments.

LGTM

github run

Merging this PR will not alter performance

✅ 70 untouched benchmarks
⏩ 129 skipped benchmarks¹

_{Comparing yagiz/improve-wpt-harness-3 (98c1425) with main (de3f468)}