Calibre does a good job of converting to both formats from HTML. I cannot remember all the specifics on how the converted ePub and FB2 looked on my Libre and PRS300, but as I recall the first two heading levels looked good (h1 and h2) but h3 and lower looked like normal text. Both formats did well with bold, italics, bold-italics. Both did well with font sizes but I cannot remember the point ranges that worked. The ePub did well with monotype, sans serif, and serif.

I didn't try mobi as I don't think it works with either of my readers, although the Libre will display PRC.

Actually, I'll attach all the conversions so you can test them out first hand. the zip file contains ePub, FB2, Mobi, RTF, TXT, and PRC all converted from the HTML file I attached above.

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