Some people leverage on a behavioral defect in browsers whereby embedding plain data in a list structure without LI elements will induce indentation. This is illegal HTML and not recommended. Other legal methods can be used to generally guarantee indentation. Lists are nestable. Internet Explorer, Opera and Netscape 3.0 and before only allow the UL Types for UL lists, and OL Types for OL lists.
All browsers have major problems with roman numeral counting and marker rendering.
See the Browser Peculiarities section of the OL element for more details on this.