Simple HTML output does not 'Validate'
If you generate Simple HTML and run the resulting output through an HTML validator, you'll get several types of errors rendering the document "invalid."
For the Simple HTML format, we use the HTML 3.2 dtd for Simple HTML, which includes many old, deprecated tags. However, it appears that there are still some [newer] tags and attributes built in to all ePublisher output formats which don't comply with that convention (namely the font face attribute and a few others). You'll also get errors stemming from the inclusion of a namespace declaration in a non-XML document.
I made some changes that instead applied the HTML 4.01 Transitional dtd, which includes support for more recent HTML elements as well as those which have been deprecated. Sadly, it didn't solve all of the problems. I wasn't able to get rid of the namespace declaration effectively, so this information has been passed on to our development team for consideration as an application defect.