Target can't interact with the DOM to determine what HTML selector is present. You'd have to pass a parameter through the mbox to be able to Target. You could use a simple javascript function to check the DOM if the class exists and if it does pass a parameter into the mbox. Then you could build and audience with the class value.
Another option would be to insert the javascript in your offer that would look at the DOM, once loaded, and then call another mbox in the offer.
If you have more details of what you are attempting to do. Their might be some other solutions.
