+/* implementation-based alternatives */
+td.di-b {background: #FDD} /* bmp */
+td.di-d {background: #FFD} /* rfc-1345 digraph */
+td.di-prop {background: #FED} /* proposed digraph */
+td.di-a {background: #EFD} /* ascii */
+td.di-rare {background: #EEE} /* disfavoured */
+td.di-invalid {background: #BBB} /* impossible */
+