- You've given your items fixed position, so they cannot slide.
- And you've not hidden the item you're sliding down, so it cannot be made to appear. (See also jQuery slideDown() animation not working)
- I think a span cannot slide to begin with, I've changed it to div.
HTML:
<body>
<div id="ONEdiv"><h1><span id="ONEspan">ONE</span></h1></div>
<div id="TWOdiv"><h1>TWO</h1></div>
<div id="THREEdiv"><h1>THREE</h1></div>
</body>
JS:
$(document).ready(function() {
$('#ONEdiv').slideDown('slow');
});
CSS:
h1 {
line-height:40px;
}
#ONEdiv {
display:none;
}
#ONEspan {
font-family:broadway;
font-size:80px;
color:#880303;
}