<style>
.ie .example { background-color: yellow; }
.ie7 .example { background-color: orange }
.gecko .example { background-color: gray; }
.win.gecko .example { background-color: red; }
.linux.gecko .example { background-color: pink; }
.opera .example { background-color: green; }
.konqueror .example { background-color: blue; }
.webkit .example { background-color: black; }
.chrome .example { background-color: cyan; }
.example { width: 100px; height: 100px; }
.no-js, .no_js, .nojs { display: block; }
.js { display: none; }
</style>

    <div class="span4">
        <h3 class="btn btn-block btn-large btn-primary">dataURI Selector</h3>

<style>
.datauri .example_bg {
    background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC');
}
.no-datauri .example_bg {
    background-image: url('bg_default.png');
}
.datauri .img_default, .no-datauri .img_uri {
    display: none !important;
}
</style>

<div class="example_bg">
  <img class="img_uri" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEUAAAD///+l2Z/dAAAAM0lEQVR4nGP4/5/h/1+G/58ZDrAz3D/McH8yw83NDDeNGe4Ug9C9zwz3gVLMDA/A6P9/AFGGFyjOXZtQAAAAAElFTkSuQmCC" alt="with dataURI" />
  <img class="img_default" src="img_default.png" alt="without dataURI" />
</div>

    </div>

    <div class="span4">
        <h3 class="btn btn-block btn-large btn-primary">Hi-dpi and Pixel-Ratio Selector</h3>

<style>
.img_hidpi {
    display: none;
}
.no-hidpi .example_bg {
    background-image: url('bg_default.png');
}
.hidpi .img_default, .no-hidpi .img_hidpi {
    display: none !important;
}
.retina_1x .example_bg {
    background-image: url('bg_hidpi_1x.png');
}
.retina_2x .example_bg {
    background-image: url('bg_hidpi_2x.png');
}
.retina_2x .img_default, .retina_2x .x1 {
    display: none !important;
}
.retina_1x .img_default, .retina_1x .x2 {
    display: none !important;
}

or /* http://bjango.com/articles/min-device-pixel-ratio/ */

@media
only screen and (min--moz-device-pixel-ratio: 1.5),
only screen and (-o-min-device-pixel-ratio: 3/2),
only screen and (-webkit-min-device-pixel-ratio: 1.5),
only screen and (min-device-pixel-ratio: 1.5) {
    .example_bg {
        background-image: url('bg_hidpi_1x.png');
    }
    .img_default, .x2 {
        display: none;
    }
}

@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) {
    .example_bg {
        background-image: url('bg_hidpi_2x.png');
    }
    .img_default, .x1 {
        display: none;
    }
}
</style>

<div class="example_bg">
    <img class="img_hidpi x2" src="img_hidpi_2x.png" alt="with pixel-ratio >= 2" />
    <img class="img_hidpi x1" src="img_hidpi_1x.png" alt="with pixel-ratio > 1 and < 2" />
    <img class="img_default" src="img_default.png" alt="with pixel-ratio 1 or default" />
</div>

    </div>

    <div class="span4">
        <h3 class="btn btn-block btn-large btn-primary">Media Queries (Screen Width)</h3>

<style>
@media (max-width: 767px) {
.example {
border: 2px solid purple!important;
}
}
/* or */
.minw_0 .example, .maxw_767 .example {
border: 2px solid purple!important;
}

@media (min-width: 768px) and (max-width: 979px) {
.example {
border: 2px solid green!important;
}
}
/* or */
.minw_768.maxw_979 .example {
border: 2px solid green!important;
}

@media (min-width: 1200px) {
.example {
border: 2px solid orange!important;
}
}
/* or */
.minw_1200 .example {
border: 2px solid orange!important;
}
</style>

    </div>
    <div class="span4">
        <h3 class="btn btn-block btn-large btn-primary">Media Queries (Orientation)</h3>

<style>
@media (orientation: landscape) {
.example {
border: 2px solid red!important;
}
}
/* or */
.orientation_landscape .example {
border: 2px solid red!important;
}

@media (orientation: portrait) {
.example {
border: 2px solid blue!important;
}
}
/* or */
.orientation_portrait .example {
border: 2px solid blue!important;
}
</style>

    </div>
</div>