I came across the situation the other day where I wanted to apply a CSS rule when two classes appeared in the same element. The Html generated was a div containing buttons. I wanted to apply a custom background to one of the items in the list. As you hover over each of the button elements JavaScript toggled the button-hover class. I wanted to have background image change on all of the list elements when button-hover was applied. The problem was I wanted to have a custom roll over images for each of the buttons.
It struck me the other day that it is so easy to use AJAX with ASP.NET it can make you lazy. I was working on a project where I had some radio boxes and based in the selection of the radio boxes another fields was displayed or hidden. I didn't want the page to post back so initially it though I'll just pop it in a AJAX UpdatePanel and write some code to show or hide the fields after the selection is made on the RadioButtonList.