Source : http://lanoie.net/classes/CSS/selectors_and_structure/lectures/lecture9.html
Contextual selectors provide the ability to create rules specific to certain structural circumstances. As an example, say you need strong text to appear blue but only if it's found within an h1 element.
You could use a class selector. However, adding the class attribute to every instance of the strong tag that is located inside an h1 tag is extra work. For example:
.setcolor { color: blue; }
<h1>This is <strong class="setcolor">Strong<strong> Text</h1>
