*, *::before, *::after{
    box-sizing: border-box;
}
body{
    margin: 0;
}

:root{
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size) * 0.9);
}

.board{
    width: 100vh;
    height: 100vh;
    display: grid;
    justify-content: center;
    align-content: center;
    grid-template-columns: repeat(3, auto);
    justify-items: center;
    align-items: center;
}
.cell{
    width: var(--cell-size);
    height: var(--cell-size);
    border: 1px solid black;
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
    cursor: pointer;
}

.cell:first-child,
.cell:nth-child(2),
.cell:nth-child(3){
    border-top: none;
}

.cell:nth-child(3n + 1){
    border-left: none;
}
.cell:nth-child(3n + 3){
    border-right: none;
}

.cell:last-child,
.cell:nth-child(7),
.cell:nth-child(8){
    border-bottom: none;
}
.cell.x::before,
.cell.x::after{
    background-color: black;
}

.board.x .cell:not(.x):not(.circle):hover::before,
.board.x .cell:not(.x):not(.circle):hover::after{
    background-color: lightgray;

}
.cell.x::before,
.cell.x::after,
.board.x .cell:not(.x):not(.circle):hover::before,
.board.x .cell:not(.x):not(.circle):hover::after{
    content: "";
    width: calc(var(--mark-size) * 0.15);
    height: var(--mark-size);
    position: absolute;
}
.cell.x::before,
.board.x .cell:not(.x):not(.circle):hover::before{
    transform: rotate(45deg);
}
.cell.x::after,
.board.x .cell:not(.x):not(.circle):hover::after{
    transform: rotate(-45deg);
}

.cell.circle,
.cell.x{
    cursor: not-allowed;
}


.cell.circle::before,
.cell.circle::after{
    content: "";
    position: absolute;
    border-radius: 50%;
}

.cell.circle::before{
    width: var(--mark-size);
    height: var(--mark-size);
    background-color: black;
}


.cell.circle::after{
    width: calc(var(--mark-size) * 0.7);
    height: calc(var(--mark-size) * 0.7);
    background-color: white;
}