<p class="main">
 <p class="box b1"></p>
 <p class="box b2"></p>
</p>

.main {
 position: absolute;
 top: 50%;
 left: 50%;
 width: 300px;
 height: 300px;
 transform: translate(-50%,-50%);
 -webkit-perspective: 1500;
 -moz-perspective: 1500;
}

.box {
 position: absolute;
 top: 0;
 left: 0;
 width: 300px;
 height: 300px;
 transition: all 1s;
 backface-visibility: hidden;
 border-radius: 10px;
 cursor: pointer;
}
.b1{
 background:skyblue;
}
.b2 {
 background:tomato;
 transform: rotateY(-180deg);
}

var b1 = document.querySelector(".b1");
var b2 = document.querySelector(".b2");
b1.onclick = function() {
 b1.style.transform = "rotateY(180deg)";
 b2.style.transform = "rotateY(0deg)";
}
b2.onclick = function() {
 b2.style.transform = "rotateY(-180deg)";
 b1.style.transform = "rotateY(0deg)";
}