How to specify the order of CSS classes?

The order in which the attributes are overwritten is not determined by the order the classes are defined in the class attribute, but instead where they appear in the CSS.

.myClass1 {color:red;}
.myClass2 {color:green;}

<div class="myClass2 myClass1">Text goes here</div>

The text in the div will appear green, and not red; because .myClass2 is further down in the CSS definition than .myClass1. If I were to swap the ordering of the class names in the class attribute, nothing would change.

The order of classes in the attribute is irrelevant. All the classes in the class attribute are applied equally to the element.

The question is: in what order do the style rules appear in your .css file. In your example, .basic comes after .extra so the .basic rules will take precedence wherever possible.

If you want to provide a third possibility (e.g., that it's .basic but that the .extra rules should still apply), you'll need to invent another class, .basic-extra perhaps, which explicitly provides for that.