<div id="transition"></div>

#transition {
    width: 300px;
    height: 300px;
    background-color: yellow;
    transition-property: width;        ➊
    transition-duration: 3s;           ➋
    transition-timing-function: ease;  ➌
    transition-delay: 1s;              ➍
}

#transition:hover{  ➎
    width: 600px;
}