What defines the numbering styles of lists in html pages?

You'd use an ordered list tag:

<ol>

</ol>

Inside this tag, you have <li></li> which will contain individual lists.

For example:

<ol>
    <li> This is my first point. </li>
    <li> This is my second point. </li>
    <li> This is my second point. </li>
</ol>

is displayed as:

1. This is my first point. 
2. This is my second point. 
3. This is my second point.