List indent generates invalid HTML

[stapersmaj]

When an (un)ordered list is indented (creating a sub-list), the result is invalid HTML. In short, the indent creates a nested list, but the nested list is not contained in a

but floats in between the other list items.

This returns errors when using the data in XHTML fields (for PDF export)

See also: https://stackoverflow.com/questions/5899337/proper-way-to-make-html-nested-list