Result

css

pre {/*全体の設定*/
  font-family: monospace;
  background-color: #fff;
  width: 27.5em;
  margin: 4em auto;
  padding: 0.5em;
  border-radius: .25em;
  box-shadow: 0.1em 0.1em 0.5em rgba(0, 0, 0, 0.45);
  line-height: 0;
  counter-reset: line;
}

pre span {/*行をブロック化*/
  display: inline-block;
  width:100%;
  line-height: 1.5rem;
}

pre span:before {/*疑似要素で行数を挿入。行数はcontent: counterで自動挿入*/
  counter-increment: line;
  content: counter(line);
  display: inline-block;
  border-right: 1px solid #ddd;
  padding: 0 .5em;
  margin-right: .5em;
  color: #888;
}

html

<div class="container">
<pre>
<span>def print_hi(name)</span>
<span>	puts "Hi, #{name}"</span>
<span>end</span>
<span></span>
<span>print_hi('Tom')</span>
<span>#=> prints 'Hi, Tom' to STDOUT.</span>
</pre>
</div>

via

CSS-only (dynamic) line numbers