Well but the way we're going there'll be a single browser engine used in the market!
I guess the ultimate cross-browser compatibility exists if you only have one browser
There's always Opera. It's straight-up written in Java.
https://en.wikipedia.org/wiki/Opera_(web_browser)
Opera is a Chromium-based browser using the Blink layout engine. It differentiates itself with a distinct user interface and other features.
That number will dwindle as more and more people adopt Chrome. The only reason we cared about browser compatibility when I used to work in web development was solely because certain customers were locked into using custom builds of IE and Netscape (am I showing my age?). The only people who really care about browser compatibility these days are advertisers, and they own Chrome.
People who care about compatibility is any smart company. Safari mobile is different from IE is different from firefox from chrome.
IE is already abandoning its platform for a chrome-based browser. Apple and Mozilla may not be far behind. There is no profit for those other companies in the browser business, it doesn't make sense for them to compete.
That won't repeat. There are too many developers and companies out there that will stick to cross-browser compatibility.