If you're drinking while programming, or holding a baby, or if you have a broken/amputated arm, you would rely heavily on the use of one hand. Therefore, the keyboard is the most important tool. That means you'll want to test with the keyboard to ensure interactive controls like links and buttons are focusable, and they have a visible focus style. You should always know where you are on a page using the keyboard only and you should be able to accomplish anything a mouse user can.