Well-structured documents are the most readable, the easiest to memorise and the most accessible. This structure can also be helpful for software tools which automatically extract information from these documents (for instance, search agents which automatically look for information, indexing tools, or browsers, which use this information in order to add browsing functions to the documents). HTML 4 adds many elements and structuring attributes.
In particular, never use structuring tags for the presentation. Respect the order of the titles (element «H1», then «H2», and son on...). For each new paragraph always use the «P» element, and to create lists use the «OL», «UL» and «DL» elements. The «DIV» element allows specifying the function of a paragraph (for example a navigation bar).
Ensure that the site structure is understandable as soon as possible. It will help the user to minimise unnecessary browsing operations. Similarly, the user should be given an idea of the content he can access, which should be precise enough (when the link is not explicit enough, provide a short description of its content).
Next : Use HTML elements advisedly