If you were to take the HTML portion of a very full-featured ePub file, converted it to .mobi format, and then extracted the HTML from the .mobi version, you will see that it likely will have lost some fidelity.
Contrary to what other people here might want to say, whenever you change eBook file formats you potentially, and almost always in practice, lose fidelity. In practice PG does use a path HTML -> ePub -> mobi, so mobi will "lose fidelity" in any case. But the major loss of fidelity is typically HTML -> ePub -- because the HTML has been written with an implied target of the original PG "author"s favorite desktop or laptop machine running their favorite HTML browser, with their favorite font, display sizing, display Gamma, HTML window opening size etc. Most authors don't seem to understand that they are accidentally or intentionally targeting one particular machine, which means that other "HTML" rendering machines -- including ePub and mobi -- often will not display this HTML as the implied intention. ====
...I discovered that the subject text had been converted to
<center>some text</center> by the conversion program. ==== Careful reading of the IDPF and/or HTML shows that the rendering process can include the "compiler" program. Thus for example "Kindlegen+Kindle" can be considered an HTML machine, and/or an ePub machine. The fact that Kindle doesn't support a particular style element isn't particularly important if Kindlegen *does.*
Perhaps no one else here is interested, but /I/ would be interested if anyone could provide me with more technical details about how the Mobipocket format has evolved since the Amazon acquisition.
I think many people in the Mobi dev community would be very interested in the "technical details" of the Mobipocket format but Amazon has been woefully unforthcoming and further the documentation they do provide is often woefully wrong. Thus, for example, discovering what glyphs a particular Kindle version actually supports has been tested using exhaustive testing techniques. This is about it when it comes to Amazon provided documentation (and even much of that is wrong) http://kindlegen.s3.amazonaws.com/AmazonKindlePublishingGuidelines.pdf