`<a>` tag doesn't have the behavior of a button. Inorder make it try the following code
a.btn{ -webkit-appearance: button; -moz-appearance: button; appearance: button;
text-decoration: none; color: initial; }

If you'd like to avoid hard-coding a specific design with css, but rather rely on the browser's default button, you can use the following css.

a.button { -webkit-appearance: button; -moz-appearance: button; appearance: button; }
Notice that it probably won't work on IE.