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

:root {
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size) * .9);
}

.board {
    display: none;
    width: 100vw;
    height: 100vh;
    justify-content: center;
    align-content: center;
    grid-template-columns: repeat(3, auto);
    justify-items: center;
    align-items: center;
    gap: 3px;
}

.board.show {
    display: grid;
}

.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;
}

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

.cell.x::before,
.cell.x::after,
.cell.circle::before,
.cell.circle::after {
    background-color: black;
}

.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: '';
    position: absolute;
    width: calc(var(--mark-size) * .15);
    height: var(--mark-size);
}

.cell.x::before,
.board.x .cell:not(.x):not(.circle):hover::before {
    transform: rotate(45deg);
    border-radius: 50%;
}

.cell.x::after,
.board.x .cell:not(.x):not(.circle):hover::after {
    transform: rotate(-45deg);
    border-radius: 50%;
}

.cell.circle::before,
.cell.circle::after,
.board.circle .cell:not(.x):not(.circle):hover::before,
.board.circle .cell:not(.x):not(.circle):hover::after {
    content: '';
    position: absolute;
    border-radius: 50%;
}

.cell.circle::before,
.board.circle .cell:not(.x):not(.circle):hover::before {
    width: calc(var(--mark-size));
    height: var(--mark-size);
}

.cell.circle::after,
.board.circle .cell:not(.x):not(.circle):hover::after {
    background: white;
    width: calc(var(--mark-size) * .80);
    height: calc(var(--mark-size) * .90);
}

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

.winning-message {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, .8);
    align-items: center;
    color: white;
    font-size: 5rem;
    flex-direction: column;
    justify-content: center;
}

.winning-message button {
    font-size: 3rem;
    background: whitesmoke;
    border: 2px solid black;
    padding: .25rem .5rem;
    cursor: pointer;
}

.winning-message button:hover {
    background: black;
    border-color: white;
    color: white;
}

.winning-message.show {
    display: flex;
}

.start-game.show {
    display: flex;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, .8);
    align-items: center;
    color: white;
    font-size: 2rem;
    flex-direction: column;
    justify-content: center;
}

.start-game {
    display: none;
}

#start {
    margin-top: .8rem;
    font-size: 3rem;
    background: whitesmoke;
    border: 2px solid black;
    padding: .25rem .5rem;
    cursor: pointer;
}

#start:hover {
    background: black;
    border-color: white;
    color: white;
}