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The spec should say what to set the data to (when invoked with no arguments), what to set the type to (if not specified), what 'endings' should be (if the second argument is omitted altogether), etc. Ideally I'd like the spec to have a single algorithm for when the constructor is invoked, where all values are initialized to the appropriate things, then the arguments are examined and change the values as appropriate, then a new Blob object is created with those values set and that is returned. (See http://www.whatwg.org/specs/web-apps/current-work/multipage/network.html#dom-websocket for an example of this style to spec constructors that doesn't leave things undefined.)
> what 'endings' should be (if the second argument is omitted altogether) Actually WebIDL should probably define this. However, the "a user agent MUST treat this as having a default value of "transparent"" requirement should be removed since it is covered in WebIDL.
The type default value should probably be specced in the IDL: dictionary BlobPropertyBag { DOMString type = ""; DOMString endings = "transparent"; };
(In reply to comment #1) > Actually WebIDL should probably define this. https://www.w3.org/Bugs/Public/show_bug.cgi?id=16725
Marking this fixed. http://dev.w3.org/2006/webapi/FileAPI/#dfn-Blob