Why use an attribute selector to match classes?

Question

I have found an example of responsive email templates where there are such CSS selectors such as the following:

a[class=\"btn\"]

Why is thi

Answer 1

The [] syntax is an attribute selector.

a[class="btn"]

This will select any tag with class="btn". However, it will not select 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.