BEM CSS

bem.css

/*-------------------------------------
BEM (Block Element Modifier)
-------------------------------------*/
/*
<a href="http://css-tricks.com" class="btn btn--green btn--big">
  <span class="btn__price">$9</span>
  <span class="btn__text">Big button</span>
</a>
*/

/* Block */
.btn {
  text-decoration: none;
  background-color: white;
  color: #888;
  border-radius: 5px;
  display: inline-block;
  margin: 10px;
  font-size: 18px;
  text-transform: uppercase;
  font-weight: 600;
  padding: 10px 5px;
}

/* Element */
.btn__price {
  background-color: white;
  color: #fff;
  padding-right: 12px;
  padding-left: 12px;
  margin-right: -10px; /* realign button text padding */
  font-weight: 600;
  background-color: #333;
  opacity: .4;
  border-radius: 5px 0 0 5px;
}

/* Element */
.btn__text {
  padding: 0 10px;
  border-radius: 0 5px 5px 0;
}

/* Modifier */
.btn--big {
  font-size: 28px;
  padding: 10px;
  font-weight: 400;
}

/* Modifier */
.btn--green {
  border-color: #3D9970;
  color: white;
  background-color: #3D9970;
}