function getAttributeSimple(el, attr) {
        // There is a potential for a warning to occur here if the attribute is a
        // custom attribute in IE<9 with a custom .toString() method. This is
        // just a warning and doesn't affect execution (see #21)
        return attr.value;
    }
Gets an attribute value in an environment without attribute bug

@param {Node} el
@param {Attr} attr
@return {String} an attribute value