"A blobParts Array
which MUST take any number of the following types of members, and in any order:"

This MUST is bogus. WebIDL makes sure the array's elements are of the right types.

Also, they aren't members, but array elements, per WebIDL terminology.

Suggested replacement:

"A blobParts Array
which takes any number of the following types of array elements, and in any order:"

(and s/member/array element/ elsewhere as appropriate)