Media Queries: How to target desktop, tablet, and mobile?

Don't target specific devices or sizes!

The general wisdom is not to target specific devices or sizes, but to reframe the term 'breakpoint':

• develop the site for mobile first using percentages or ems, not pixels,
• then try it in a larger viewport and note where it begins to fail,
• redesign the layout and add a CSS media query just to handle the broken parts,
• repeat the process until you reach the next breakpoint.

You can use responsivepx.com to find the 'natural' breakpoints, as in 'breakpoints are dead' by Marc Drummond.

Use natural breakpoints

The 'breakpoints' then become the actual point at which your mobile design begins to 'break' i.e. cease to be usable or visually pleasing. Once you have a good working mobile site, without media queries, you can stop being concerned about specific sizes and simply add media queries that handle successively larger viewports.

If you're not trying to pin design to exact screen size, this approach works by removing the need to target specific devices. The integrity of the design itself at each breakpoint ensures that it will hold up at any size. In other words, if a menu/content section/whatever stops being usable at a certain size, design a breakpoint for that size, not for a specific device size.

See Lyza Gardner's post on behavioral breakpoints, and also Zeldman's post about Ethan Marcotte and how responsive web design evolved from the initial idea.

Use semantic markup

Further, the simpler and more semantic the DOM structure with nav, header, main, section, footer etc. (avoiding abominations like div class="header" with nested inner div tags) the easier it will be to engineer responsiveness, especially avoiding floats by using CSS Grid Layout (Sarah Drasner's grid generator is a great tool for this) and flexbox for arranging and re-ordering according to your RWD design plan.

IMO these are the best breakpoints:

@media (min-width:320px)  { /* smartphones, portrait iPhone, portrait 480x320 phones (Android) */ }
@media (min-width:480px)  { /* smartphones, Android phones, landscape iPhone */ }
@media (min-width:600px)  { /* portrait tablets, portrait iPad, e-readers (Nook/Kindle), landscape 800x480 phones (Android) */ }
@media (min-width:801px)  { /* tablet, landscape iPad, lo-res laptops ands desktops */ }
@media (min-width:1025px) { /* big landscape tablets, laptops, and desktops */ }
@media (min-width:1281px) { /* hi-res laptops and desktops */ }

Edit: Refined to work better with 960 grids:

@media (min-width:320px)  { /* smartphones, iPhone, portrait 480x320 phones */ }
@media (min-width:481px)  { /* portrait e-readers (Nook/Kindle), smaller tablets @ 600 or @ 640 wide. */ }
@media (min-width:641px)  { /* portrait tablets, portrait iPad, landscape e-readers, landscape 800x480 or 854x480 phones */ }
@media (min-width:961px)  { /* tablet, landscape iPad, lo-res laptops ands desktops */ }
@media (min-width:1025px) { /* big landscape tablets, laptops, and desktops */ }
@media (min-width:1281px) { /* hi-res laptops and desktops */ }

In practice, many designers convert pixels to ems, largely because ems afford better zooming. At standard zoom 1em === 16px, multiply pixels by 1em/16px to get ems. For example, 320px === 20em.

In response to the comment, min-width is standard in "mobile-first" design, wherein you start by designing for your smallest screens, and then add ever-increasing media queries, working your way onto larger and larger screens.

Regardless of whether you prefer min-, max-, or combinations thereof, be cognizant of the order of your rules, keeping in mind that if multiple rules match the same element, the later rules will override the earlier rules.