Základ pro piškvorky, kód od Aleše - děkuji. 

<!DOCTYPE HTML SYSTEM>
<html>
<head>
<style>
table {border-collapse:collapse}
 
 td {
 border:2px solid black;
 padding:10px;
 }
</style>
<title>
Piškvorky
</title>
</head>
Piškvorky
<br>
<br>
 
<body>
<table>
<tr>
<td id="bunka1_1"></td>
<td id="bunka1_2"></td>
<td id="bunka1_3"></td>
</tr>
<tr>
<td id="bunka2_1"></td>
<td id="bunka2_2"></td>
<td id="bunka2_3"></td>
</tr>
<tr>
<td id="bunka3_1"></td>
<td id="bunka3_2"></td>
<td id="bunka3_3"></td>
</tr>
</table>
 
<script>
var naTahu="X";
 
function kliknuto(){
 // this.style.backgroundColor="green";
 this.innerHTML=naTahu;
 
 if(naTahu=="X"){
  naTahu="O";
 }else if(naTahu=="O"){
  naTahu="X";
 }  
 
}
 
//1.Řada
document.getElementById("bunka1_1").onclick = kliknuto;
document.getElementById("bunka1_2").onclick = kliknuto;
document.getElementById("bunka1_3").onclick = kliknuto;
//2.Řada
document.getElementById("bunka2_1").onclick = kliknuto;
document.getElementById("bunka2_2").onclick = kliknuto;
document.getElementById("bunka2_3").onclick = kliknuto;
//3. Řada
document.getElementById("bunka3_1").onclick = kliknuto;
document.getElementById("bunka3_2").onclick = kliknuto;
document.getElementById("bunka3_3").onclick = kliknuto;
</script>
</body>
</html>