explanation #1
Description
I have the same issue. My workaround is to inject a tag with appropriate styles around my HTML fragment.
The apple docs claim that NSDefaultAttributesDocumentAttribute is for "plain files":
NSDefaultAttributesDocumentAttribute
@"DefaultAttributes", NSDictionary containing attributes to be applied to plain files. Used by reader methods. This key in options can specify the default attributes applied to the entire document contents. Upon return, the document attributes can contain this key indicating the actual attributes used.
My guess is this means it can only be used when importing with NSPlainTextDocumentType.