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/* To install, copy the following line to your Special:MyPage/common.css
@import url('https://en.wikipedia.org/w/index.php?action=raw&title=User:Anomie/listgap.css&ctype=text/css');
*/

/* Show a dotted red line wherever a [[WP:LISTGAP]] occurs. */
ol + ol, ol + ul, ol + dl,
ul + ol, ul + ul, ul + dl,
dl + ol, dl + ul, dl + dl {
    border-top: 1px dotted red;
}