Here’s an example which works in all browsers. The pure CSS approach works in the real browsers (i.e. everything but IE6/7) and the jQuery code is to cover the unsupported. It’s in flavor of an SSCCE, you can just copy’n’paste’n’run it without changes.
<!doctype html>
<html lang="en">
<head>
<title>SO question 2729927</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).ready(function() {
if ($('ol:first').css('list-style-type') != 'none') { /* For IE6/7 only. */
$('ol ol').each(function(i, ol) {
ol = $(ol);
var level1 = ol.closest('li').index() + 1;
ol.children('li').each(function(i, li) {
li = $(li);
var level2 = level1 + '.' + (li.index() + 1);
li.prepend('<span>' + level2 + '</span>');
});
});
}
});
</script>
<style>
html>/**/body ol { /* Won't be interpreted by IE6/7. */
list-style-type: none;
counter-reset: level1;
}
ol li:before {
content: counter(level1) ". ";
counter-increment: level1;
}
ol li ol {
list-style-type: none;
counter-reset: level2;
}
ol li ol li:before {
content: counter(level1) "." counter(level2) " ";
counter-increment: level2;
}
ol li span { /* For IE6/7. */
margin: 0 5px 0 -25px;
}
</style>
</head>
<body>
<ol>
<li>first</li>
<li>second
<ol>
<li>second nested first element</li>
<li>second nested second element</li>
<li>second nested third element</li>
</ol>
</li>
<li>third</li>
<li>fourth</li>
</ol>
</body>
</html>