It is very much easier to apply CSS when we have ul,li elements as the HTML content. If we look into ASP.NET Menu Control till Version 3.5, its render as Table-TR-TD Tag. Though Table/Tr/Td is quite useful to display tabular data but sometime creates a big problem when we need to do more work with CSS. To overcome this problem we generally used CSS Friendly adapter to render the ASP.NET Control in ul/li mode.
ASP.NET 4.0 makes the things easier for web developer by providing "RenderingMode" properties. Here we can specify RenderMode of a ASP.NET Menu control. Which define the what will be the HTML Render Content Type. Bydefault mode is "List" which means control will be render as ul/li.
Read Complete Article @ Abhijit's World Of .NET
CSS Friendly Menu Control in ASP.NET 4.0