A list of input values for which this element should be shown.
Multiple values can be separated by either a space (foo bar) or a comma (foo, bar).
If your values might contain spaces, you may also serialize them as a relaxed JSON array (["foo", "bar"]).
To react to the presence or absence of a value, use :blank or :present.
For checkboxes, you can react to :checked or :unchecked in addition
to the checked value.
