HTML:

<a href="#" class="btn">button</a>
<a href="#" class="btn-rounded">button</a>
<a href="#" class="btn-small">button</a>
<a href="#" class="btn-small-rounded">button</a>
<a href="#" class="btn-large">button</a>
<a href="#" class="btn-large-rounded">button</a>


CSS:

[class^=btn] {
  display: inline-block;
  font-size: 1.2em;
  line-height: 1.75; 
  text-decoration: none;
  color: white;
  padding: 0 1.5em;
  background-color: green;
}

[class^=btn-small] { font-size: .8em }
[class^=btn-large] { font-size: 1.5em }
[class$=rounded] { border-radius: .3em }