exo2.css

*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

body{
    width: 100%;
    height: 100%;

}
.container{
    width:  100%;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: -1;
    
}

 .btn{
    width: 20%;
    height: 100px;
    position: absolute;
    border: 2px solid gray;
    justify-content: center;
    align-items: center;
    display: flex;
    overflow: hidden;
}
    
 .btn::after{
    content: "";
    width: 100%;
    height: 100%;
    background-color:red;
    position: absolute;
    top: 0;
    right: 0;
    transform: rotate(90deg); 
    transform-origin: bottom right;
    transition: 1s;
    z-index: -1;
    
 }

.btn::before{
    content :"";
    width: 100%;
    height: 100%;
    background-color: red;
    position: absolute;
    display: block;
    top: 0;
    left: 0;
    transform: rotate(-90deg);
    transform-origin: bottom left;
    transition: 1s;
    z-index: -1;
}

.btn:hover::after{
    transform: rotate(0deg);
}

.btn:hover::before{
    transform: rotate(0deg);
}