тег
<body> забыли
<!DOCTYPE html>
<body></body>
<script type = "text/javascript">
var table = document.createElement("TABLE");
for(var r = 0; r < 10; r ++){
var row = table.insertRow(-1);
for(var c = 0; c < 10; c ++){
var cell = row.insertCell(-1);
cell.innerHTML = r + "-" + c;
cell.style.border = "1px solid red";
}
}
document.body.appendChild(table);
</script>