align items left after flex-direction row reverse
#main {
width: 400px;
height: 400px;
border: 1px solid #c3c3c3;
display: flex;
flex-direction: row-reverse;
align-items: flex-start;
justify-content: flex-end; <------need this
}
#main div {
width: 50px;
height: 50px;
}