gtk2/testsuite/css/parser/enum.css

a {
  enum-property: in;
}

b {
  enum-property: none;
}

c {
  enum-property: 2;
}

d {
  enum-property: "in";
}

e {
  enum-property: non-existing;
}

f {
  enum-property: "non-existing";
}

g {
  enum-property: 34;
}

h {
  enum-property: 12e;
}