pre.number-lines{
    counter-reset: line-number;
}

pre.number-lines > span.line::before{
    border-right: 0.5em solid indigo;
    content: counter(line-number);
    counter-increment: line-number;
    display: inline-block;
    margin-right: 0.5em;
    padding-right: 0.5em;
    text-align: right;
    width: 2ch;
}