The strings do not have to be quoted to be considered for aggregation. Any string that does not contain non-quoted whitespaces will do.
But is there any _point_ in aggregating a quoted string with a non-quoted one? And, again, was such behaviour ever documented?
a="foo"bar='gazonk' would define the attribute 'a' with the value "foobar=gazonk" (this was one of the actually tested cases).
But there is no incentive for _not_ writing this as
a="foobar=gazonk".
It just doesn't make any sense.
Is there an example of something which is valid HTML _and_ which would (for good reason) be interpreted as something else by the old parser?
Well, the one above. And, "for good reason" is relative, I guess.
If you present the reason why anyone would write that, then I might be able to judge whether it is "good" or not...
I think the intended usecase was this:
foo="'"bar"'" to set the attribute "foo" to "'bar'"
^^^^^^^ But you just said how that is written shorter and less convoluted!