I just wanted to add a couple of points. Don't trust the Kindle Previewer. It usually works ok for the Kindle Fire modes, but all of the rest of the modes are broken in various ways.
The KF8 HTML/CSS renderer is based on the Webkit browser. That's a fact. There are lots of features that are undocumented by Amazon, but work just fine. In my production process, I prefer to run the book's text through a hyphenation algorithm and insert soft hyphens (which doesn't require jailbreaking, it will work on any KF8 capable Kindle), but it doesn't surprise me that -webkit CSS feature works.