Ability to add CSS classes to elements
When styling elements with CSS, elements have to be targeted hierarchically. so as to avoid other elements which have different styles and contexts (but the same class) picking up the new css. This often means there's elements that are not obvious as well as inside the edit view that pick up styling when you don't want them to, which creates more reliance on QA..
The ability to add a css class to an element if one doesn't exist (or append a class if there already is a class attribute) that gets added to the html would allow users to create classes for things like calls to actions which although currently have the same class could have vasty different uses and contexts.
For example, on the styling of the element in edit view you could have a "define css class" text input which allows you to enter a class name of your choosing which would get added to the HTML. So the HTML would look something like this: <element tag class="added class gets added or appended here" />