pre{
    counter-reset: line;
}

code{
    counter-increment: line;
}

code:before{
    content: counter(line);
    width: 3em;
    border-right: 1px solid #ddd;
    padding: 0 .5em;
    margin-right: .5em;
    color: #888;
    -webkit-user-select: none;
}