You should
NOT rely on Calibre's ebook viewer to preview/test an ebook conversion, particularly MOBI conversions.
To properly preview/text a MOBI file you should use either a Kindle device or the Kindle Previewer. The latter is a special software viewer that is designed to replicate the display on the device. The current version can replicate the K3, KDXG, Kindle for iPob, & Kindle for iPhone and is available free from this page:
http://www.amazon.com/gp/feature.htm...cId=1000234621
Also, the
<PRE>...</PRE> tag set specifies that all contents are pre-formatted. You can't reliably include any other HTML formatting tags inside of
<PRE> section. When you do, they should be displayed by the browser/reader/viewer.