https://heycam.github.io/webidl/#es-dictionary

At step 5.1.2, both undefined and null are treated as missing, but in Blink's dictionary_v8.cpp, only undefined can lead to the TypeError being thrown. If null is passed, instead null will simply be passed along. This means that event constructor dictionaries with required members of interface type don't work as intended, and the resulting event instances will return null for nominally non-nullable attributes. Example:

new BlobEvent('foo', {data:null}).data (is null, should throw TypeError)