Why use an attribute selector to match classes?
The [] syntax is an attribute selector.
a[class="btn"]
This will select any <a> tag with class="btn". However, it will not select <a> which has class="btn btn_red", for example (whereas a.btn would). It only exactly matches that attribute.
You may want to read The 30 CSS Selectors you Must Memorize. It's invaluable to any up-and-coming web developer.