[npaun]

Adds a WPT target for html/webappapis/structured-clone: 117 of 135 subtests pass and 18 are expected failures.

• The ImageBitmap and OffscreenCanvas subtests are omitted because workerd has no canvas APIs.

The expected failures:

• The enhanced_error_serialization compat flag copies an error's custom own properties, and gives an Error with an empty message its own message property. The spec does neither, but workerd does this on purpose.
• File can't be cloned, although Blob can.
• structuredClone(new Response()) succeeds; the spec requires a DataCloneError.
• Only ArrayBuffers can be transferred. Transferring a MessagePort or ReadableStream throws a TypeError, and so does transferring a Blob, which should be a DataCloneError.
• Transferring an ArrayBuffer that is already detached succeeds instead of throwing.
• "Growable SharedArrayBuffer" fails because of a bug in the test: it checks a SharedArrayBuffer with instanceof ArrayBuffer, which is never true.

[clanky-the-clanker]

💬 Approved with comments — The test configuration is clean and every expected failure is explained. One optional comment-wording suggestion remains.

Findings: 1 suggestion

_{The decision is advisory: this review is a comment and never approves, blocks or requests changes on the pull request.}

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📝 Documentation: 💬 1 suggestion

One wording suggestion remains: the comment does not say the transfer limitation is specific to workerd.

Recommendation: 💬 Optional Nits

• [SUGGESTION] Say that the transfer limitation is specific to workerd (e.g. "workerd's structuredClone can only transfer ArrayBuffers"). As written, the comment suggests the standard API only supports transferring ArrayBuffers. (src/wpt/html/webappapis/structured-clone-test.ts:45)

View full review

• src/wpt/html/webappapis/structured-clone-test.ts:45-46: "structuredClone can only transfer ArrayBuffers" can be read as a statement about the API in general. The standard also allows transferring MessagePorts and ReadableStreams. Saying "workerd's structuredClone" would make it clear that this is a workerd limitation.

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✅ All other checks passed

📋 Code Quality: ✅ LGTM

The new html/webappapis/structured-clone target in src/wpt/BUILD.bazel is correct. The module is listed as both wpt_directory and extra_wpt_directories on purpose: build/wpt_test.bzl removes the duplicate from data and still binds the module under its absolute path. The tests' META: script=/html/webappapis/structured-clone/... lines need that path. In the test config, the battery helper files are omitted and every expected failure has a comment explaining it.

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_{Reviewed commit: 1c630d35}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Files                           ?      726           
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  Hits                            ?   129568           
  Misses                          ?    42719           
  Partials                        ?    17375           

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