You can do this, use data- attributes though (part of the HTML5 specification), like this:
<li data-something="CommonLi"></li>
jQuery even has built-in support for these in 1.4.3+, for example:
$("li").data("something") //"CommonLi"
For your other questions:
- They’ll validate if it’s HTML5 – but won’t break anything in HTML4
- This shouldn’t interfere with the designer, but it’ll depend on which designer
- If you’re fetching from an element, the performance is the same as any other attribute