css - Retina displays, high-res background images

Question

This might sound like a silly question.

If I use this CSS snippet for regular displays (Where box-bg.png is 200px by 200px);

.box{
    background:url('images/box-bg.png') no-repeat top left;
    width:200px;
    height:200px
}

and if I use a media query like this to target retina displays (With the @2x image being the high-res version);

@media (-webkit-min-device-pixel-ratio: 2), 
(min-resolution: 192dpi) { 

    .box{background:url('images/box-bg@2x.png') no-repeat top left;}
}

Do I need to double the size of the .box div to 400px by 400px to match the new high res background image?

Answer 1

Do I need to double the size of the .box div to 400px by 400px to match the new high res background image

No, but you do need to set the background-size property to match the original dimensions:

@media (-webkit-min-device-pixel-ratio: 2), 
(min-resolution: 192dpi) { 

    .box{
        background:url('images/box-bg@2x.png') no-repeat top left;
        background-size: 200px 200px;
    }
}

---

EDIT

To add a little more to this answer, here is the retina detection query I tend to use:

@media
only screen and (-webkit-min-device-pixel-ratio: 2),
only screen and (   min--moz-device-pixel-ratio: 2),
only screen and (     -o-min-device-pixel-ratio: 2/1),
only screen and (        min-device-pixel-ratio: 2),
only screen and (                min-resolution: 192dpi),
only screen and (                min-resolution: 2dppx) { 

}

- Source

NB. This min--moz-device-pixel-ratio: is not a typo. It is a well documented bug in certain versions of Firefox and should be written like this in order to support older versions (prior to Firefox 16). - Source

---

As @LiamNewmarch mentioned in the comments below, you can include the background-size in your shorthand background declaration like so:

.box{
    background:url('images/box-bg@2x.png') no-repeat top left / 200px 200px;
}

However, I personally would not advise using the shorthand form as it is not supported in iOS <= 6 or Android making it unreliable in most situations.