text-wrap: pretty tells the browser to wrap the text so as to make it look pretty. But the CSS standard doesn't specify what exactly that means; it's up to each individual browser to decide what algorithm yields the prettiest results.

Chromium is the only browser engine whose stable channel currently supports text-wrap: pretty. In this post, WebKit is announcing not only that they've implemented it (though not yet in a stable channel), but that they've done so using an algorithm that's better than Chromium's. Their algorithm adjusts for various things that Chromium's currently does not.