Returns the value of the given attribute on the given element, cast as a boolean value.
If the attribute value cannot be cast to true or false, undefined is returned.
Casting rules
This function deviates from the
HTML Standard for boolean attributes
in order to allow undefined values. When an attribute is missing, Unpoly considers the value to be undefined
(where the standard would assume false).
Unpoly also allows "true" and "false" as attribute values.
The table below shows return values for up.element.booleanAttr(element, 'foo') given different elements:
