counter-increment

<style type="text/css">
<!-- 
h2:before {content: counter(i) ". "; counter-increment: i; counter-reset: j;}
h3:before {content: counter(i) "." counter(j) " "; counter-increment: j;}
-->
</style>

...

<h2>Text</h2>
<h3>Text</h3>
<h3>Text</h3>
<br>
<h2>Text</h2>
<h3>Text</h3>
<h3>Text</h3>