.grid {
    width: 300px;
    height: 300px;
    border: solid 1px black;
    display: flex;
    flex-wrap: wrap

}

.grid div {
    width: 20px;
    height: 20px;
    /*border: solid 1px red;*/
    /*box-sizing: border-box;*/

}

.invader {
    background-color: purple;
    border-radius: 10px;
}

.shooter {
    background-color: green;

}

.laser {
    background-color: orange;
}

.boom {
    background-color: red;
}