document.getElementsByTagName("td") returns a nodelist, not a single element. That means you are setting the onclick property on the wrong element (the nodelist instead of the actual elements).

If there always is only one element returned, you can use document.getElementsByTagName("td")[0].onclick, but you would probably want to loop over the elements, like this:

tds = document.getElementsByTagName("td");
for( var x=0; x < tds.length; x++ ) {
 tds[x].onclick = function(){
 alert('Click!');
 };
}

Thank you, that makes sense.

For testing, I changed the script from td to h3, as there is only one of those and set the script to:

<script type="text/javascript">
document.getElementsByTagName("h3")[0].onclick=function(){alert("click");}
</script>

Unfortunatly, this doesn't seem to work either.

Do I need to "turn on" this thing or will JavaScript always be watching out for the onclicks, or do I need to put it in something like onload?

Thanks

or do I need to put it in something like onload

Yes you need to wait for document to load first.