alt131 - 11:55 am on Feb 10, 2012 (gmt 0) [edited by: alt131 at 7:34 am (utc) on Feb 11, 2012]
:active applies when an element is has been activated - for example, by a mouse press, and it ceases to apply on mouse-up or when the mouse is moved off. A mouse click (or tabbing) gives the element focus, which is a different state that applies to elements that accept user input (such as <input> and <a>, and remains in place until another element is clicked or tabbed. Read more in 5.11.3 The dynamic pseudo-classes: :hover, :active, and :focus [w3.org] I'm not sure I've explained that well, so it might help to watch them in action at son of spring's demo page [sonspring.com].
In that way :active is similar to a "touch" and I understand some devices are using it as the "hover equivalent" - while others have chosen to just apply the :hover rule. Setting the same rule for :active and :hover will have limited impact on mouse or keyboard users, and means the menu should close once the element ceases to be active.
I think the "a#" you are referring to is the href attribute of a link ( <a> ). (Tell me if I have that wrong). That is different to the dynamic pseudo-classes, so you can set li:hover and li:active even though there is isn't an <a> or href.
Correcting smile face
[edited by: alt131 at 7:34 am (utc) on Feb 11, 2012]