Trent Walton with some great advice about getting started with responsive web design. I’m glad Trent included this part about content-based breakpoints:
It’s good to plan, design, and test with all sorts of devices in mind, but it’s become standard to let media queried breakpoints be defined by content/layout rather than any specific device. Whenever things don’t fit or hierarchy breaks, add a media query.
See also: ‘There is no breakpoint’.