ok, <!doctype html><html>
<head>
<meta charset="utf-8">
<title>EsercizioPerCasa</title>
<style type="text/css">
.csstab{
font-size: 50px;/*Grandezza font*/
font-family: skia;
background-color: #dddddd;

}

</style>
<script type="text/javascript">

function myFunction(p_id) {
var x=document.getElementById(p_id);
x++;
p_id.innerHTML=""+x;
}
</script>
</head>
<body>
<header>
</header>
<nav>
</nav>
<aside>
</aside>
<section>
<article>
</article>


<div align="center">
<table>
<tr class="csstab">
<td id="1" onclick="myFunction(p_id)"> 1 </td>
<td id="2"> 2 </td>
<td id="3"> 3 </td>
</tr>
<tr class="csstab">
<td id="5"> 4 </td>
<td id="6"> 5 </td>
<td id="7"> 6 </td>
</tr>
<tr class="csstab">
<td id="7"> 7 </td>
<td id="8"> 8 </td>
<td id="9"> 9 </td>
<tr>
</table>
<p id="p_id"></p>


ecco qui succede che se premo nella cella 1 spunta soltanto 1