+can have are 1, -1, and empty. Clicking the checkbox cycles between them.
+
+For compatibility with regular checkboxes, empty is the false state and
+-1 is the indeterminate state.
+
+Displaying these is a problem. "indeterminate" is a standard HTML5 attribute
+but some browsers display it in unhelpful ways (e.g. Firefox slightly grays
+the checkbox, approximately #dddddd), and checkboxes ignore nearly all CSS
+styling.