The <menu> tag displays a list of short choices to the reader, such as a menu of links to other documents. The browser may use a special (typically more compact) representation of items in a menu list compared with the general unordered list, or even use some sort of graphical pull-down menu to implement [...]

The <dir> Tag
The designers of HTML originally dedicated the <dir> tag for displaying lists of files. As such, the browser, if it treats <dir> and <ul> differently at all (most don’t), expects the various list elements to be quite short, possibly no longer than 20 characters or so. Some browsers display the elements in [...]

Except inside directories or menus, lists nested inside other lists are fine. Menu and directory lists can be embedded within other lists.
Indents for each nested list are cumulative, so take care not to nest lists too much; the list contents could quickly turn into a thin ribbon of text flush against the right [...]