The reason for this is that scripts in the head load before the page is rendered. This means your content is not yet rendered and therefore not a part of document.
If you want to see this work, try moving your script below the element renders, like this:
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<div id="ssio"></div>
<script type="text/javascript">
var ssio = document.getElementById('ssio');
ssio.innerHTML = "it finally works!";
</script>
</body>
</html>
A more standardized way of doing this is with events. Many people use jQuery but it can be done with plain js. This would mean changing your script like this:
<script type="text/javascript">
function WinLoad() {
var ssio = document.getElementById('ssio');
ssio.innerHTML = "It finally works!";
}
window.onload = WinLoad;
</script>
This way you can still leave it in the <head>.
Also, using .html is from jQuery. It is generally used as .html(content). If you want to use the plain javascript version use .innerHTML = content.
I mention jQuery so much because it is a highly used API. This quote is from their site:
jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.
