div{
    width: 90%;
    height: 200px;
    margin: 0 auto;
    border: 2px solid black;
    box-sizing: border-box;
}

/*Etat normal*/
#tr1{
    background-color: orange;
    transition-property: background-color;
    transition-duration: 5s;
}

/*Lors du survol*/
#tr1:hover{
    background-color: blue;
}