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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div style="width:0;height:0;overflow:hidden;">
          <svg>
            <linearGradient id="btn-grad" x1="0%" y1="-20%" x2="250%" y2="20%">
              <stop offset="0%" style="stop-color:#FD8BA9;stop-opacity:1" />
              <stop offset="25%" style="stop-color:#9A94E8;stop-opacity:1" />
              <stop offset="50%" style="stop-color:#2EDBF8;stop-opacity:1" />
              <stop offset="75%" style="stop-color:#F5F1C8;stop-opacity:1"/>
              <stop offset="100%" style="stop-color:#B1E9F9;stop-opacity:1" />
            </linearGradient>
  </svg>
</div>

<svg width="0" height="0" id="morph-button-states" preserveAspectRatio="none" viewBox="0 0 105.45 53.14">
<path class="default" d="M82.7,46.1h-60c-11,0-20-9-20-20c0-11,8.9-20,20-20h60c11,0,20,9,20,20C102.7,37.1,93.8,46.1,82.7,46.1z"/>
<path class="over" d="M102.7,26.1c0,20.4-11.5,26.1-50,26.1c-37.2,0-50-6.9-50-26.1C2.7,6.9,14,0,52.7,0S102.7,7.6,102.7,26.1z"/>
<path class="down" d="M82.8,43.2c-20-2-40.2-1.9-60.1,0.2c-11,1.5-19.9-5.8-19.9-17.3c0-11.5,8.9-18.8,19.9-17.3c20,2.2,40.2,2.2,60.1,0.2 c11-1.4,19.9,5.8,19.9,17.1C102.7,37.4,93.8,44.6,82.8,43.2z"/>
</svg>


<button><span>I'm a button!</span></button>
!

CSS 

              
                body{
  padding: 6em;
  text-align: center;
}
button{
  padding: 2em;
  font-size: 20px;
  border: 0;
  font-weight: bold;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  position: relative;
  background: none;
  color: white;
  outline: 0;
  cursor: pointer;
  transition: all .1s ease-in;
  
}
button>span{
  position: relative;
}
button>svg{
  position: absolute;
  left:0;
  right:0;
  top:0;
  bottom:0;
  width: 100%;
  height: 100%;
  display: block;
  fill: url(#btn-grad);
}
button:active,
button:hover:active{
  transform: scaleX(1.1);
}
button:hover{
  transform: scaleY(1.05);
}
!

JS 

              
                function MorphButton( btn, svg ){

    if( !(this instanceof MorphButton) ){
      new MorphButton( btn, svg );
      return;
    }

    function getObjects(selector){
      var objs = [];
      if( typeof selector == 'string' ){
        objs = document.querySelectorAll(selector);
      } else {
        if( 'forEach' in selector ){
          objs = selector;
        } else {
          objs = [selector];
        }
      }
      return objs;
    }

    var btnObjects = getObjects(btn),
        svgObject = getObjects(svg)[0];


    var btnStates = {
      default: svgObject.querySelector('path:nth-child(1)').getAttribute('d'),
      over: svgObject.querySelector('path:nth-child(2)').getAttribute('d'),
      down: svgObject.querySelector('path:nth-child(3)').getAttribute('d')
    };

    function initBtn(b){

      //insert an svg/path
      var s = document.createElementNS("http://www.w3.org/2000/svg", "svg");
      s.setAttribute('preserveAspectRatio',svgObject.getAttribute('preserveAspectRatio'));
      s.setAttribute('viewBox', svgObject.getAttribute('viewBox'));
      var path = document.createElementNS(s.namespaceURI,'path');
      s.appendChild(path);
      path.setAttribute('d',btnStates.default);
      s.setAttribute('fill','black');

      b.insertBefore(s,b.firstChild);

      //rig it to animations
      b.addEventListener('mouseenter',function(){
        TweenLite.to( path, .3, { morphSVG: btnStates.over, ease: Bounce.easeOut });
      });
      b.addEventListener('mouseleave',function(){
       TweenLite.to( path, .3, { morphSVG: btnStates.default, ease: Bounce.easeOut });
      });
      b.addEventListener('mousedown',function(){
        TweenLite.to( path, .3, { morphSVG: btnStates.down, ease: Bounce.easeOut });
      });
      b.addEventListener('mouseup',function(){
        TweenLite.to( path, .3, { morphSVG: btnStates.over, ease: Bounce.easeOut });
      });

    }

    //inject the svg
    btnObjects.forEach(initBtn);
};

MorphButton('button','#morph-button-states');
!
999px

Console