If an element is well formed AND valid it won't have any weird effects, just whatever you code on itt.
If you have a well formed element and it ISN'T valid, there is no telling what will happen-with invalid code you can't expect valid results
If you have a non well formed element and ISN'T valid you are just asking for trouble. No telling what will happen.
If you have a non well formed element and IS valid still no tellinng what will happen.