如何判断浏览器是否处于“怪癖”模式?

Let's suppose you have a page with a relatively strict doctype and HTML markup that's pretty close to compliant, but perhaps misses in a few silly ways, perhaps because of user content that's out of your control... say you're working on a content management system or a theme for a content management system where you control some basic structure and need some javascript, but you're not responsible for everything else that goes into pages.Let's suppose you have a page with a relatively