Is it true that border-top-left-radius only accepts one value? In that case, you can animate it like this:
$('.class').animate({borderTopLeftRadius: 20})
UPDATE
I'm not sure if jQuery natively supports this. You might need to implement the step callback like so:
$('#container').animate({
borderTopLeftRadiusSideLength: 500,
borderTopLeftRadiusUpperLength: 50
}, {
duration: 5000,
step: function (val, context) {
$(this).data(context.prop, val);
var side = $(this).data('borderTopLeftRadiusSideLength');
var upper = $(this).data('borderTopLeftRadiusUpperLength');
if (side && upper) {
$(this).css('borderTopLeftRadius', side + 'px ' + upper + 'px');
}
}
});
Check out this jsFiddle link for more details.