According to the standard, custom attributes _must_ begin with 'data-'. For example, `data-a-block`. Of course, it still works if you omit 'data-', so I fully e

According to the standard, custom attributes _must_ begin with 'data-'. For example, `data-a-block`. Of course, it still works if you omit 'data-', so I fully expect that this requirement will be changed to "must contain a hyphen".