style.css

*{
    margin: 0;
    box-sizing: border-box;
    height: 100vh;
    padding: 0;
}
:root{
    --cell-size: 100px;
    --mark-size: calc(var(--cell-size)*.9);
}
body{
    display: flex;
    justify-content: center;
    align-items: center;
}
.gameboard{
    display: flex;
    flex-flow: column wrap;
    justify-content: center;
    height: 300px;
    width: 300px;
}
.square{
    height: var(--cell-size);
    width: var(--cell-size);
    border: 1px solid black;
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
}
.square:first-child,
.square:nth-child(4),
.square:nth-child(7) {
    border-top: none;
}

.square:nth-child(2),
.square:nth-child(3),
.square:nth-child(1)
{
    border-left:none;
}

.square:nth-child(3),
.square:nth-child(8),
.square:nth-child(6),
.square:nth-child(9)
{
    border-bottom: none;
}

.square:nth-child(9),
.square:nth-child(8),
.square:nth-child(7){
    border-right: none;
}

/* cross css */
.square.x::before,
.square.x::after{
    position: absolute;
    content: "";
    height: var(--mark-size);
    width: calc(var(--mark-size) * .15);
    background-color: black;
}
.square.x::before
{
    transform: rotate(45deg);
}
.square.x::after{
    transform: rotate(-45deg);
}

/* circle css */
.square.circle{
    background-color: aqua;
}

.square.circle::before,
.square.circle::after{
    position: absolute;
    content: "";
    border-radius: 50%;
}
.square.circle::before{
    background-color: black;
    width: var(--mark-size);
    height: var(--mark-size);
}
.square.circle::after{
    background-color: white;
    width: calc(var(--mark-size)* .7);
    height: calc(var(--mark-size)* .7);
}

/* not allowed twice to click on same cell */
.square.circle,
.square.x{
    cursor: not-allowed;
}

.reset{
    width: 50px;
    height: 50px;
}
.startgame{
    width: 50px;
    height: 50px;
}