body {
    margin:0;
    width:100vw;
    height:100vh;
    display: grid;
    grid-template-rows: 0.1fr 1fr 1fr;
    grid-template-columns: 1fr 1fr;
}

h1 {
    grid-column: 1 / 3;
    text-align: center;
}

section {
    display: grid;
    grid-template-rows: 0.1fr 1fr;
    padding:5px;
}

section h2 {
    text-align: center;
}

section {
    border-top: 1px solid black;
}

section:first-of-type, section:nth-of-type(3) {
    border-right: 1px solid black;
}


/* EXERCICE 1 */

#ex1 div {
    width: 15vh;
    height: 15vh;
    background-color: red;
    opacity: 0;
}


/* EXERCICE 2 */
#ex2 div {
    width: 15vh;
    height: 15vh;
    background: linear-gradient(red, blue);
    border-radius: 50%;
}


/* EXERCICE 3 */
#ex3 div {
    width: 15vh;
    height: 15vh;
    background: linear-gradient(red, blue);
    border-radius: 50%;
}


/* EXERCICE 4 */
#ex4 div {
    width: 15vh;
    height: 15vh;
    background: linear-gradient(red, blue);
    border-radius: 50%;
}