There you go - blame the Browser. As I mentioned, we prefer Firefox, especially for designing Courses. IE seems to work OK in compatibility mode; and you should also test your Courses in Chrome, Safari and even Opera. The HTML editor responds a little differently in each one - so stick with Firefox for stability.
In www.W3Schools.com you can find a table that lists incompatibilities among the Browsers. Well written HTML usually contains variations to accomodate these differences.