Originally Posted by mrmikel
The above may not necessarily be true if you have used calibre to create the text.
One of the objections to Calibre, as wonderful as it is, is that in order to achieve cross platform compatibility, it does things in complicated ways. Instead of simply using bold or italic, it creates whole new styles and series of styles. Then it throws in a number of <div class="bs2">s, for example, in order to display ordinary text.
These cascading styles can make it appear that things don't work, when in fact there is an active style which already covers the paragraph in question. This could be the cause of your troubles, as the rendering engines of calibre and Sigil are not the same as in the readers or Reader Library or Kindle for PC.
One other possibility is that reader software may not make any attempt to guess if your code is defective, even to the wrong case being used. Sigil and Calibre's rendering engines I think make more of an attempt to guess, and may work, even though the code is wrong.
Garbage in, garbage out. Calibre is only as good as the garbage input to it. A lot of Mobipocket eBooks are full of garbage code and thus, Calibre gives similar code to the ePub. I've never had Calibre convert such that an italics/bold did not work as it should have regardless of how messy the code was.