Quick and dirt alternative solution. You can use a tabulation character along with preformatted text. Here's a possibility:
<style type="text/css">
ol {
list-style-position: inside;
}
li:first-letter {
white-space: pre;
}
</style>
and your html:
<ol>
<li> an item</li>
<li> another item</li>
...
</ol>
Note that the space between the li tag and the beggining of the text is a tabulation character (what you get when you press the tab key inside notepad).
If you need to support older browsers, you can do this instead:
<style type="text/css">
ol {
list-style-position: inside;
}
</style>
<ol>
<li><pre> </pre>an item</li>
<li><pre> </pre>another item</li>
...
</ol>