Suppose you have a CSS style defined as class="myClass" as shown below:
<div class ="myClass">
<!-- HTML code -->
Now suppose you want to know exactly how the myClass style definition is defined.
Just place your cursor on myClass and press F12. This will highlight the CSS class for you whether the class definition is in the current page or in an external style sheet.
Basically, F12 is a shortcut key you can use instead of right-clicking a method or class and selecting "Go to Definition" from the context menu.