It’s important to remember that long selectors incur a small rendering performance penalty that in aggregate can slow down your web page. There is no need to exactly mimic your document structure in your css. Instead nest only deep enough that the selector is unique to that part of the document. For instance, don’t use table thead tr th when a simple th selector will suffice. This might mean that you have to separate your styles into several selectors and let the document cascade work to your advantage.
摘自：http://compass-style.org/help/tutorials/best_practices/
长的选择器会影响渲染表现，累积起来会导致页面变慢。
没有必要在css里效仿你的文档结构。只需要嵌套到该选择器在该部分文档唯一的就足够了。
比如，当一个简单的th选择器就够了的时候，不要使用table thead tr th。
这意味着你必须把样式分散在几个选择器中。