<style>
ol li::before { content: counter(l1) ":"; counter-increment: l1;}
ol li { list-style-type: decimial; display: block; }
</style>

<div id="container" style="counter-reset:l1;">
<ol style="list-style-type:decimal">
<li id="line1">line1
<li id="line2">line2
<li>line3
</ol>
</div>