I'm not seeing the HTML export on the desktop version of Kindle (from 2015) on my work laptop at the moment, but I'm positive it's hiding somewhere on my desktop machine, which is currently out of commission for some quirky problems. When I get it running, I'll try to document the feature as I don't think I've seen it mentioned anywhere else and it was tremendously easy and helpful.
I hadn't tried Bookcision as I'd stumbled across what I supposed was an easy answer, but there are a few edge cases for non-Amazon books in my collection that might benefit from using it as Amazon doesn't always play well with content that doesn't originate from the "mothership".
A while back, I'd bookmarked http://the-digital-reader.com/2015/02/21/how-to-download-your-kindle-notes-and-highlights-and-export-them/ as one of the better articles explaining most of the pathways currently available. If I remember the comments were relatively helpful as well.
For general file conversion you might appreciate Pandoc: http://pandoc.org/
There are also some in the community who have tools (like Kevin Marks' http://www.unmung.com/ ) which can do inter-conversion. I can't recall specifics at the moment, but someone in the chat will likely know of the others, or you can search github to find some of the opensource ones https://github.com/search?q=indieweb&ref=opensearch (it can be an interesting browse in general for things you didn't know you needed/wanted).
Finally, I liked the teleprompter.css hint for marked2, but if you've not come across it, I often read newspaper, magazine articles, and fiction on my mobile using RSVP-related technology (Rapid Serial Visual Presentation) which has been patented (though I'm not so sure it's enforcable) by http://spritzinc.com/get-spritz and for which there are multiple apps floating around for multiple platforms. My favorite thus far is Balto-reader which I think may only be available in the Amazon store, but it's one of the few I've found that can break through Amazon's encryption to allow using Spritz-like technology on my Kindle Fire or Android Phones. I don't recommend using it to read technical documents or passages which require more thought, but it's lovely on mobile and has a nice bookmarklet for reading web-based articles quickly. If it's something you think you'd use, but you can't find something that can access Amazon books, let me know and I can suggest some workarounds that will allow Amazon book access.