button.js

window.addClass = function(el, className) {
  if (el.classList) el.classList.add(className);
  else el.className += " " + className;
};

window.removeClass = function(el, className) {
  if (el.classList) el.classList.remove(className);
  else
    el.className = el.className.replace(
      new RegExp("(^|\\b)" + className.split(" ").join("|") + "(\\b|$)", "gi"),
      " "
    );
};

window.busy = function() {
  var that = this;
  addClass(this, "busy");
  setTimeout(function() {
    that.disabled = true;
  }, 0);
};

window.removeBusy = function() {
  removeClass(this, "busy");
  this.disabled = false;
}

buttons.css

button {
    position: relative;
    border: none;
    border-radius: 4px;
    outline: none;
    text-decoration: none;
    color: #fff;
    background: #32325d;
    white-space: nowrap;
    display: inline-block;
    height: 40px;
    line-height: 40px;
    padding: 0 14px;
    box-shadow: 0 4px 6px rgba(50, 50, 93, .11), 0 1px 3px rgba(0, 0, 0, .08);
    border-radius: 4px;
    font-size: 15px;
    font-weight: 600;
    letter-spacing: 0.025em;
    text-decoration: none;
    -webkit-transition: all 150ms ease;
    transition: all 150ms ease;
}

button:hover {
    transform: translateY(-1px);
    box-shadow: 0 7px 14px rgba(50, 50, 93, .10), 0 3px 6px rgba(0, 0, 0, .08);
    background-color: #43458b;
}

button:after {
    position: absolute;
    left: calc(50% - 0.5em);
    top: 25%;
    height: 1em;
    width: 1em;
    content: '';
    opacity: 0;
    line-height: 100%;
    border-left: 2px solid rgba(255, 255, 255, 0.2);
    border-right: 2px solid rgba(255, 255, 255, 0.2);
    border-bottom: 2px solid rgba(255, 255, 255, 0.2);
    border-top: 2px solid rgba(255, 255, 255, 0.8);
    border-radius: 100%;
    -webkit-animation: rotate 0.6s infinite linear;
    -moz-animation: rotate 0.6s infinite linear;
    -ms-animation: rotate 0.6s infinite linear;
    -o-animation: rotate 0.6s infinite linear;
    animation: rotate 0.6s infinite linear;
    -moz-transition: opacity 0.15s ease-out;
    -o-transition: opacity 0.15s ease-out;
    -webkit-transition: opacity 0.15s ease-out;
    transition: opacity 0.15s ease-out;
}

button.busy {
    color: transparent;
}

button.busy:after {
    opacity: 1;
}

@keyframes rotate {
    from {
        transform: rotate(0deg);
    }
    to {
        transform: rotate(360deg);
    }
}