How to code CSS media queries targeting ALL mobile devices and tablets?

This can be done with Level 4 Media Queries: (Browser Support is quite good - CaniUse)

Interaction Media Features

The idea here is to target devices based on their capabilities. (as apposed to say, checking the size or resolution of the device which tend to be moving targets)

The pointer media feature queries the quality of the pointer mechanism used by the device.

The hover media feature queries the user’s ability to hover over elements on the page with a given device.

So to answer the question...

Mobile devices/tables are similar in that:

1) The accuracy of the primary input mechanism of the device is limited.

This means we could use the following media query:

@media (pointer:coarse) {
    /* custom css for "touch targets" */
}

div {
  width: 400px;
  height: 200px;
  color: white;
  padding: 15px;
  font-size: 20px;
  font-family: Verdana;
  line-height: 1.3;
  background: green;
}
@media (pointer:coarse) { 
  div {
    background: red;
  }
}

<h2>The <a href="https://www.w3.org/TR/mediaqueries-4/#pointer">pointer media feature</a> queries the quality of the pointer mechanism used by the device.</h2>
<div>The background here will be green on 'desktop' (devices with an accurate pointing mechanism such as a mouse) and red on 'mobile' (devices with limited accuracy of primary input mechanism) </div>

Codepen Demo

2) The primary pointing system can’t hover

So our media query would look like this:

@media (hover: none) { 
   /* custom css for devices where the primary input mechanism cannot hover 
   at all or cannot conveniently hover
}

NB: Chrome/Android prior to version 59 required the on-demand value for hover/any-hover media queries. This value was later removed from the spec and no longer required by Chrome from version 59. So to support older versions of Android you need

@media (hover:none), (hover:on-demand) { 
  /* custom css for "touch targets" */
}

div {
  width: 400px;
  height: 150px;
  color: white;
  padding: 15px;
  font-size: 20px;
  font-family: Verdana;
  line-height: 1.3;
  background: green;
}
@media (hover:none), (hover:on-demand){ 
  div {
    background: red;
  }
}

<h2>The <a href="https://www.w3.org/TR/mediaqueries-4/#hover">hover media feature</a> queries the user’s ability to hover over elements on the page</h2>
<div>The background here will be green on 'desktop' (devices which support hover) and red on 'mobile' (devices which don't [easily] support hover ) </div>

Codepen Demo

NB:

Even if you were to connect a mouse to a mobile/tablet, the hover media feature still matches none since their primary interaction mode doesn't support hover.

If we do want to take secondary devices into consideration we could use the any-pointer and any-hover features

So if we wanted mobile devices connected with a pointing device to be treated like a 'desktop' we could use the following:

@media (any-hover: hover) { ... }

Extra reading

• Interaction Media Features and their potential (for incorrect assumptions)
• https://css-tricks.com/touch-devices-not-judged-size/

Edit Note: This is specifically a method that worked with older browsers. The accepted answer has been updated to a more modern CSS that does have media queries that makes this easy. I mainly suggest using my older code for CSS specific to older browsers.

Instead of using specific widths initially, or messing around with orientations, or any other nonsense, I suggest using the following media tag...

@media only screen and (min-resolution: 117dpi) and (max-resolution: 119dpi), only screen and (min-resolution: 131dpi) and (max-resolution: 133dpi), only screen and (min-resolution: 145dpi) and (max-resolution: 154dpi), only screen and (min-resolution: 162dpi) and (max-resolution: 164dpi), only screen and (min-resolution: 169dpi) {
  /* Your touch-specific css goes here */
}
@media only screen and (min-resolution: 165dpi) and (max-resolution: 168dpi), only screen and (min-resolution: 155dpi) and (max-resolution: 160dpi), only screen and (min-resolution: 134dpi) and (max-resolution: 144dpi), only screen and (min-resolution: 120dpi) and (max-resolution: 130dpi), only screen and (max-resolution: 116dpi) {
  /* Your click-specific css goes here */
}

And what do you use these tags for? To set stuff for hover & click vs touch events.

Touch devices, other than a few devices in grey areas above addressed, have a very different resolution than desktop devices. Do -not- this to set design elements, but navigation elements. Some pudents may cry that some insanity with max-width may be better, but there's so many HD phones it's ridiculous that device-max-width quickly becomes useless.

However, you should use width media width queries. However, don't bother with max-device-width, just max-width & min-width. Let the above tags address your touch vs not touch users, let min-width vs max-width address based on window size and adjust site visuals.

Further, using orientation to determine if it's mobile or not is just silly, as even monitors can be placed in various orientations (a common setup I've seen for 3-monitors is a portrait center monitor and landscape side monitors.)

For width views, focus on making your site clean on various widths, ignoring what kind of device is actually accessing it, just make sure your screen displays cleanly at various sizes. That's good design that applies to both desktop and mobile. If they have your site in a small window at the upper left corner of their screen for reference (or quick distraction) while using the majority of their screen real estate elsewhere, and it should be for them, as well as mobile users, that your smaller widths are built for. Trying anything else is quickly going down a very painful and self-defeating path for web development. So for those smaller widths, you can set your widths to whatever you want, but I'll include a few I personally like.

So altogether, you should have something like this...

@media only screen and (min-resolution: 117dpi) and (max-resolution: 119dpi), only screen and (min-resolution: 131dpi) and (max-resolution: 133dpi), only screen and (min-resolution: 145dpi) and (max-resolution: 154dpi), only screen and (min-resolution: 162dpi) and (max-resolution: 164dpi), only screen and (min-resolution: 169dpi) {
    #touch_logo {
       display: inherit;
    }
    #mouse_logo {
       display: none;
    }
  /* Your touch-specific css goes here */
}
@media only screen and (min-resolution: 165dpi) and (max-resolution: 168dpi), only screen and (min-resolution: 155dpi) and (max-resolution: 160dpi), only screen and (min-resolution: 134dpi) and (max-resolution: 144dpi), only screen and (min-resolution: 120dpi) and (max-resolution: 130dpi), only screen and (max-resolution: 116dpi) {
    #touch_logo {
       display: none;
    }
    #mouse_logo {
       display: inherit;
    }
  /* Your click-specific css goes here */
}
@media (min-width: 541px){
  /* Big view stuff goes here. */
}
@media (max-width: 540px) and (min-width: 400px){
  /* Smaller view stuff goes here. */
}
@media (max-width: 399px){
  /* Stuff for really small views goes here. */
}

Although, don't forget to include the following in your page's head:

<meta name='viewport' content="width=device-width, initial-scale=1" />

It may still break on some cases, but this should be more concise and more complete than many other solutions.

You have your main desktop styles in the body of the CSS file (1024px and above) and then for specific screen sizes:

@media all and (min-width:960px) and (max-width: 1024px) {
  /* put your css styles in here */
}

@media all and (min-width:801px) and (max-width: 959px) {
  /* put your css styles in here */
}

@media all and (min-width:769px) and (max-width: 800px) {
  /* put your css styles in here */
}

@media all and (min-width:569px) and (max-width: 768px) {
  /* put your css styles in here */
}

@media all and (min-width:481px) and (max-width: 568px) {
  /* put your css styles in here */
}

@media all and (min-width:321px) and (max-width: 480px) {
  /* put your css styles in here */
}

@media all and (min-width:0px) and (max-width: 320px) {
  /* put your css styles in here */
}

This will cover pretty much all devices being used - I would concentrate on getting the styling correct for the sizes at the end of the range (ie 320, 480, 568, 768, 800, 1024) as for all the others they will just be responsive to the size available.

Also, don't use px anywhere - use em's or %