The compatible problem is Microsoft and Safari, not Chromium and Firefox. I run a web development agency, and supporting the limitations IE has always been a significant cost and challenge. As the joke goes, "What is the definition of HTML5? If it works in IE, it's not HTML5".

I have mixed feelings about Microsoft giving up on their browser. On the one hand, it will make things more compatible and reduce effort for us. On the other, it's one step closer to a monoculture controlled by Google. I would prefer that Microsoft simply get their shit together and make a good browser.