The syntax for WebIDL2JSValueAsUnsupported is invalid

[saschanaz]

interface URLSearchParamsCollection {
  [WebIDL2JSValueAsUnsupported=null] getter URLSearchParams? namedItem(DOMString name);
};

Here, null is tokenized as a terminal symbol null rather than an identifier per the grammar:

If the longest possible match could match one of the above named terminal symbols or one of the other terminal symbols from the grammar, it must be tokenized as the latter.

And thus cannot be parsed as:

ExtendedAttributeIdent ::
    identifier = identifier

Currently webidl2.js 13+ fails to parse it, so this should be fixed before upgrading to the latest version.

[saschanaz]

The easiest workaround would be adding the escape character _ before null.

[saschanaz]

By the way, I think [WebIDL2JSValueAsUnsupported=null] is not needed as ? already means nullable per the spec.