Device breakpoints 是什么意思？求解

[annawishing]

今天看了篇国外关于 CSS 的文章，里面提到个 Device breakpoints ，不知道怎么理解啊。

原文内容如下：
A few months ago, in the wake of Apple ’ s latest Chinese factory exorcism, I read a tweet by a commentator. Their tweet contained a diagram of, “ all the viewports available on iOS 9 for Responsive Web Design ”.
What? What?
Then I recalled a conversation I ’ d had with Sara Soueidan, in which she described content breakpoints as responsive design ’ s “ biggest secret ”.