Originally Posted by William Ockham
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.
Please, don't add the soft hyphens if you are planning on selling these eBooks. All it is going to do is bloat the size of the KF8 and possibly screw up the Mobi. Most people won't be bothered by not having the soft hyphens. And given that the PW has less memory then previous models, it's just wasting that memory to have the soft hyphens.
Of course, if they are for your own personal use, that's fine for you to do whatever it is that you like.