Wrong font size appearing

I'm converting an old WinHelp context sensitive help system into MS HTML Help. The standard format calls for the title for each page to be in 12 pt Arial and the text body to be in 10 pt Arial. When any page is accessed directly from the host application, the entire page appears in 12 pt, which usually messes up the layout and makes it difficult to read. However, if a page is accessed from the Help system TOC, the format appears correctly.

This phenomenom appears with both Windows XP and Vista, so it doesn't appear to be an operating system problem. I've examined the html code and can't see anything obviously wrong.