Double border with different color

With Photoshop, I can put two different border to an element with two different color. And with that, I can make many dynamic shade-effect with my elements. Even with Photoshop effects, I can manage that with Drop Shadow and Inner Shadow.

On the Web Design concern, if I have design like the image below, how can I achieve that with CSS? Is it really possible?

NOTE: I'm giving two borders to the white element: the outer border is white, and the inner border is greyish. Together, they create a dynamic look so that it feels like an inset element, and the white element is pillow embossed. So thing is a bit:

div.white{
   border: 2px solid white;
   border: 1px solid grey;
}

But you know it's a double declaration, and is invalid. So how can I manage such thing in CSS?

And if I put border-style: double then you know I can't pass two different color for the singe double border.

div.white{
       border: double white grey;
}

Additionally, I'm familiar with LESS CSS Preprocessor. So if such a thing is possible using CSS Preprocessor, please let me know.

Alternatively, you can use pseudo-elements to do so :) the advantage of the pseudo-element solution is that you can use it to space the inner border at an arbitrary distance away from the actual border, and the background will show through that space. The markup:

body {
  background-image: linear-gradient(180deg, #ccc 50%, #fff 50%);
  background-repeat: no-repeat;
  height: 100vh;
}
.double-border {
  background-color: #ccc;
  border: 4px solid #fff;
  padding: 2em;
  width: 16em;
  height: 16em;
  position: relative;
  margin: 0 auto;
}
.double-border:before {
  background: none;
  border: 4px solid #fff;
  content: "";
  display: block;
  position: absolute;
  top: 4px;
  left: 4px;
  right: 4px;
  bottom: 4px;
  pointer-events: none;
}

<div class="double-border">
  <!-- Content -->
</div>

If you want borders that are consecutive to each other (no space between them), you can use multiple box-shadow declarations (separated by commas) to do so:

body {
  background-image: linear-gradient(180deg, #ccc 50%, #fff 50%);
  background-repeat: no-repeat;
  height: 100vh;
}
.double-border {
  background-color: #ccc;
  border: 4px solid #fff;
  box-shadow:
    inset 0 0 0 4px #eee,
    inset 0 0 0 8px #ddd,
    inset 0 0 0 12px #ccc,
    inset 0 0 0 16px #bbb,
    inset 0 0 0 20px #aaa,
    inset 0 0 0 20px #999,
    inset 0 0 0 20px #888;
  /* And so on and so forth, if you want border-ception */
  margin: 0 auto;
  padding: 3em;
  width: 16em;
  height: 16em;
  position: relative;
}

<div class="double-border">
  <!-- Content -->
</div>

I use outline a css 2 property that simply works. Check this out, is simple and even easy to animate:

.double-border {
  display: block;
  clear: both;
  background: red;
  border: 5px solid yellow;
  outline: 5px solid blue;
  transition: 0.7s all ease-in;
  height: 50px;
  width: 50px;
}
.double-border:hover {
  background: yellow;
  outline-color: red;
  border-color: blue;
}

<div class="double-border"></div>

you can add infinite borders using box-shadow using css3 suppose you want to apply multiple borders on one div then code is like:

div {
    border-radius: 4px;
    /* #1 */
    border: 5px solid hsl(0, 0%, 40%);
    
    /* #2 */
    padding: 5px;
    background: hsl(0, 0%, 20%);
    
    /* #3 */
    outline: 5px solid hsl(0, 0%, 60%);
    
    /* #4 AND INFINITY!!! (CSS3 only) */
    box-shadow:
        0 0 0 10px red,
        0 0 0 15px orange,
        0 0 0 20px yellow,
        0 0 0 25px green,
        0 0 0 30px blue;
}

Use of pseudo-element as suggested by Terry has one PRO and one CON:

1. PRO - great cross-browser compatibility because pseudo-element are supported also on older IE.
2. CON - it requires to create an extra (even if generated) element, that infact is defined pseudo-element.

Anyway is a great solution.

---

OTHER SOLUTIONS:

If you can accept compatibility since IE9 (IE8 does not have support for this), you can achieve desired result in other two possible ways:

1. using outline property combined with border and a single inset box-shadow
2. using two box-shadow combined with border.

Here a jsFiddle with Terry's modified code that shows, side by side, these other possible solutions. Main specific properties for each one are the following (others are shared in .double-border class):

.left
{
  outline: 4px solid #fff;
  box-shadow:inset 0 0 0 4px #fff;
}

.right
{
  box-shadow:0 0 0 4px #fff, inset 0 0 0 4px #fff;
}

---

LESS code:

You asked for possible advantages about using a pre-processor like LESS. I this specific case, utility is not so great, but anyway you could optimize something, declaring colors and border/ouline/shadow with @variable.

Here an example of my CSS code, declared in LESS (changing colors and border-width becomes very quick):

@double-border-size:4px;
@inset-border-color:#fff;
@content-color:#ccc;

.double-border 
{
  background-color: @content-color;
  border: @double-border-size solid @content-color;
  padding: 2em;
  width: 16em;
  height: 16em;
  float:left;
  margin-right:20px;
  text-align:center;
}

.left
{
  outline: @double-border-size solid @inset-border-color;
  box-shadow:inset 0 0 0 @double-border-size @inset-border-color;
}

.right
{
  box-shadow:0 0 0 @double-border-size @inset-border-color, inset 0 0 0 @double-border-size @inset-border-color;
}

Maybe use outline property

    <div class="borders">
      Hello
    </div>

    .borders{
    border: 1px solid grey; 
    outline: 2px solid white;
     }

https://jsfiddle.net/Ivan5646/5eunf13f/

You can use outline with outline offset

<div class="double-border"></div>
.double-border{
background-color:#ccc;
outline: 1px solid #f00;
outline-offset: 3px;
}

Try below structure for applying two color border,

<div class="white">
    <div class="grey">
    </div>
</div>

.white
{
    border: 2px solid white;   
}

.grey
{
    border: 1px solid grey;   
}

You can use the border and box-shadow properties along with CSS pseudo elements to achieve a triple-border sort of effect. See the example below for an idea of how to create three borders at the bottom of a div:

.triple-border:after {
    content: " ";
    display: block;
    width: 100%;
    background: #FFE962;
    height: 9px;
    padding-bottom: 8px;
    border-bottom: 9px solid #A3C662;
    box-shadow: -2px 11px 0 -1px #34b6af;
}

<div class="triple-border">Triple border bottom with multiple colours</div>

You'll have to play around with the values to get the alignment correct. However, you can also achieve more flexibility, e.g. 4 borders if you put some of the attributes in the proper element rather than the pseudo selector.