BR=0;
el=false;
incr=0;
function doOneLoop(x,i)
{
    x.style.borderRadius=BR+'%';
    BR+=i;
    incr=(BR<5) ? 5 : i;
    if (BR<55) tout=setTimeout('doOneLoop(el,incr)',200);
 }
function doOne()
{
    el=document.getElementById('fx1');
    BR=50;
    incr=-5;
    tout=setTimeout('doOneLoop(el,incr)',100);
}

http://coboldinosaur.com/pages/shapes-art.html

Changes shape back and forth from a round element to a square element.

Animating Shapes

css, html

JavaScript