A PR will be needed, ideally before stage 2.7 (although this would otherwise be a concern to be addressed within 2.7) to ensure that DOM exceptions are considered to be Errors.

Some potential implementation strategies:

1. add the ErrorData internal slot to DOM exceptions, and accept the implicit observable change when used with Object.prototype.toString and a deleted Symbol.toStringTag
2. add a new host hook, that the web can use to indicate which of its errors are Errors
3. optionally, check the host hook in Object.prototype.toString so that host errors are indistinguishable from true Error subclasses