Buttons are intended for action and thus should have appropriate weight. The standard button is given that weight with a little bit of depth and a strong hover.

Click Me

Code Example
<!-- Just add .button to an anchor -->
<a href="#" class="button">Click Me</a>
<!-- Also works on plain button elements -->
<button>Click Me</button>
<!-- To take up full-width of a container? -->
<a href="#" class="full-width button">Click Me</a>