CSS与所有浏览器通用兼容？

I thought this kind of CSS was universally compatible with all browsers?

不适用。 IE（直到并包括版本6）忽略链中除最后一个之外的所有类选择器。 IE9可能会在您的网页quirks模式，这意味着它会像其前身一样，将 .drop.a 选择器视为 .a 。

Nope. IE up to and including version 6 ignores all class selectors in a chain except the last one. IE9 may be rendering your page in quirks mode, meaning it'll act like its predecessors, and treat your .drop.a selector as .a instead.

尝试在页面中添加一个doctype声明，看看是否可以修复它。如果您的信息页上已有某个信息，但您仍然看到此行为，则会出现其他错误。

Try adding a doctype declaration to your page and see if that fixes it. If you already have one on your page and you're still seeing this behavior, something else is wrong.