This test:

https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/external/wpt/WebCryptoAPI/encrypt_decrypt/aes_gcm_vectors.js?q=aes_gcm_vectors.js&dr=C&l=249

Expects AES-GCM to fail with an OperationError when given a tag length of 256.
However in Chrome, it fails with a TypeError.

I believe Chrome's error is the correct one, since:
    [EnforceRange] octet tagLength;

Therefore a tagLength of 256 is outside of the range of octet, and should be an IDL level failure.

Here is what the test currently gives in Chrome:

FAIL AES-GCM 128-bit key, illegal tag length 256-bits assert_equals: Should throw an OperationError instead of AesGcmParams: tagLength: Outside of numeric range expected "OperationError" but got "TypeError"