The KF8 format is currently only used on the Kindle Fire and (perhaps, reported to work - but undocumented) on Kindle for PC. In the long term it is going to be the best Kindle alternative to ePub, but for now for the majority of Kindle readers the original MOBI format is what you need.
MOBI does not include embedded fonts and has limited CSS support (there is no CSS in the MOBI, but some ePub CSS wil get converted to the equivalent MOBI formatting). So the "simple" online previewer may be accurately showing you the MOBI version.
If I was producing a Kindle ebook today via Linux, I would use Calibre to produce a MOBI from the ePub. If the MOBI looks good in the Calibre viewer it will probably be ok on a Kindle (and in the "simple" online previewer). A "good" MOBI should have a cover and a metadata-based TOC and maybe some "frills" for chapter headings (although this are more limited for MOBI than for ePub). Italics should be maintained from the ePub, but this assumes you used HTML to get the italics (using an embedded italics-only font isn't good practice in an ePub and won't convert to a MOBI).