I formatted my book and it came out perfectly fine in the Kindle Fire previewer, but when I opted to check for the regular Kindle, a couple of the chapter headings had bold type (instead of italic). I checked the code (and I know only the basics of code), and it was marked properly with the html "i" code..
Since the text shows up correctly in the Kindle Fire preview, should I assume that this is a more advanced viewer and so more representative of any Kindle device out there?
I would rather have the text perfect, if possible.
Kindle books contain two separate versions of the book, one in the older Mobi7 format and one in the newer Kindle Format 8 (KF8). The default device mode in Kindle Previewer (Kindle Fire) displays the latter, while the other modes (along with the online previewer) display the former.
It is possible for these two versions to differ significantly in their final presentation, and is in fact quite common for books converted from Word documents or HTML that has not been properly prepared for conversion.
Since both types of readers (Mobi7 and KF8) are still used in large numbers, which is unlikely to change in the foreseeable future, you will want to ensure that the formatting appears as intended in both versions. It is also a good idea to test on as wide a range of devices as possible, since some formatting will appear differently depending on the device being used, even if it is displaying the same version of the book.
Something else to keep in mind is that headings (HTML tags <h1>, <h2>, etc.) will [i]always[/i] be bolded in the Mobi7 version. If you would like headings that do not appear in bold you would want to use a paragraph with appropriate styling for size, text indentation, and alignment.