Web guidelines for content editors: Lists

HTML lists – <ul>, <ol>, and <dl> – also convey a hierarchical content structure. Each of these has rules regarding their use as well. Unordered lists should be used when there is no order of sequence or importance. Ordered lists suggest a progression or sequence. Definition lists should be used explicitly for presenting a structure for definitions. As with heading, lists should be used correctly and for the right purposes. Unordered and ordered lists should always contain list items. Definition lists must always have definition descriptions. Empty lists are incorrect HTML. Lists should never be used for merely indenting or other layout purposes. Nested lists should be coded properly. [from WebAIM]