This isn't a big deal, but it was something I was fighting with for a bit. If you're trying to set the properties of a DIV via javascript, and you're getting the "document.getElementById("mydiv") has no properties" javascript warning, there's probably two big things that will cause it.
1) you didn't set the id.
Go back and make sure your div has id="mydiv" (or whatever you named it) in there.
2) the javascript is running BEFORE the div has been defined in the code.
This is what got me. I had the javascript code just above where the div was... and at load time, that div doesn't exist yet... the browser hasn't gotten that far down the page yet, so as far as it's concerned, that div with that id, doesn't exist. Once I moved the javascript BELOW the div... all was well.
And actually here - they suggest calling it on onload.