The default behavior of numbers in WebIDL is to wrap around.

For instance consider this WebCrypto parameter:
   unsigned long modulusLength;

If you were to specify a negative number for modulusLength then the spec says it should work, but silently convert to a positive number.

This seems like a bad pattern. It is likely that providing a negative number in a place which expected an unsigned number was a user error, and hence should trigger a failure.

I propose marking all numeric fields as [EnforceRange] so the user agent will throw a TypeError whenever the number is outside of the expected range.