Let's always use jQuery methods for DOM manipulation, even if only for consistency.

Regarding boolean values: Afaik it expects those "booleans" as strings, as they're not the only valid values. Specifically, `aria-invalid` accepts "true", "false" and "spelling"(!). [Listed here](http://www.w3.org/TR/wai-aria/states_and_properties#aria-invalid).

jQuery UI does this accordingly, [e.g. to set aria-disabled](https://github.com/jquery/jquery-ui/blob/fe9f6ca98634dd82c3cdb907d725c42f97afe951/ui/jquery.ui.menu.js#L63).