The jQuery .data() documentation says: "Every attempt is made to
convert the string to a JavaScript value (this includes booleans,
numbers, objects, arrays, and null) otherwise it is left as a
string. To retrieve the value's attribute as a string without any
attempt to convert it, use the attr() method."
(imported from commit f47c1cbb94cb5a98ea9842b00f45c35cd21873f9)