3-1 響應式斷點

Bootstrap4 為行動優先的架構，所以預設的斷點為 (xs)

// Extra small devices (portrait phones, less than 576px)
// No media query since this is the default in Bootstrap
 
// Small devices (landscape phones, 576px and up) (sm)(橫的手機)
@media (min-width: 576px) { ... }
 
// Medium devices (tablets, 768px and up) (md)(平板)
@media (min-width: 768px) { ... }
 
// Large devices (desktops, 992px and up) (lg)(桌機)
@media (min-width: 992px) { ... }
 
// Extra large devices (large desktops, 1200px and up) (xl)(較大的桌機)
@media (min-width: 1200px) { ... }