It has an animated transparent widget.

THIS ONE

<button class="cos-btn-transparent-white"
        style="width:208px;
               height:64px;
               font-size:27.2px;
               border-width:3px;
              ">
        THIS ONE
</button>

Also other colored transparent buttons.

BLACK BLUE
GREEN RED
GREY GOLD

DISABLED

<button class="cos-btn-transparent-black">BLACK<button>
<button class="cos-btn-transparent-darkblue">BLUE<button>
<button class="cos-btn-transparent-darkgreen">GREEN<button>
<button class="cos-btn-transparent-darkred">RED<button>
<button class="cos-btn-transparent-grey">GREY<button>
<button class="cos-btn-transparent-gold">GOLD<button>
<button disabled class="cos-btn-transparent-darkred">DISABLED</button>

And has some material-styled ones.

DARK PINK
GREEN BLUE
RED ORANGE
GREY SKY

DISABLED

<button class="cos-btn-material-dark">DARK</button>
<button class="cos-btn-material-pink">PINK</button>
<button class="cos-btn-material-green">GREEN</button>
<button class="cos-btn-material-blue">BLUE</button>
<button class="cos-btn-material-red">RED</button>
<button class="cos-btn-material-orange">ORANGE</button>
<button class="cos-btn-material-grey">GREY</button>
<button class="cos-btn-material-sky">SKY</button>
<button disabled class="cos-btn-material-green">DISABLED</button>

CUSTOMIZED

<button class="cos-btn-material"
        style="width:144px;
               background:rgb(209, 236, 59);
               color:black;
        ">
        CUSTOMIZED
</button>