One might think is would be a prerequisite, and I would agree.
But whilst working locally I hadn't declared a doctype:
Make sure you declare one:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<meta http-equiv="x-ua-compatible" content="IE=8" >
Jobs a good'n