Question about

Based on the content of the stage three proposal, we ended up with the following paragraph in the spec: The roles of an alternative title are specified by the @title-role attribute.
Multiple roles can be specified, separated by white space.
An alternative title must specify at least one role.
The @title-role attribute is not required by the grammar files.
Should it be?
If not, what does An alternative title must specify at least one role mean?

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Best, Kris