no CSSStyleValue subclass yet.
// <transform-function> has no CSSStyleValue subclass yet.

// StylePropertyMap.append

// It is not allowed to append CSSStyleValues to custom properties, even
// when the string matches the syntax of the custom property.

test_specified_maps(function(map){
    verify_map_no_append(map, generate_property('*'), 'a');
}, 'Appending a string to * is not allowed');

test_specified_maps(function(map){
    verify_map_no_append(map, generate_property('foo+'), 'foo');
}, 'Appending a