.input {
    float: left;
    margin-right: 10px;
    width: 223px;
}

@media screen and (max-width: 720px) {
    #comments {
        display: inline;
        display: inline-block;
        width: 100%;
    }
    .input {
        float: none;
        width: 100%;
    }
    .contenu {
        margin-bottom: 60px;
    }
}