So we need `data-` attributes on custom elements? Might be cleaner to use unprefixed if that is valid. None of the Polymer/X-Tags examples I've seen use `data-`.

Also `data-action` is kind of ambiguous. A header (unlike `<form>`) can perform more than one 'action'. Something more like `back`, `back=true` or `primary-action="back"` might make more sense?