In Bootstrap 2, we have added mobile device friendly styles to some key parts of the framework. In Bootstrap 3, we rewrote the whole framework, making it initially friendly to mobile devices. This time, instead of simply adding some optional styles for mobile devices, it directly merges into the kernel of the framework. That is to say, Bootstrap is the (44)点击定位 of mobile devices. Style integration for mobile devices goes into every corner of the framework, rather than adding an additional file.