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CSS

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CSS Base

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JavaScript

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HTML 

              
                <!-- https://css-tricks.com/the-layered-look/ -->

<section class="page-wrap">
  
  <h1>The Layered Look - <span class="nobr">Faking "lowsrc"</span></h1>
  <h2>Better Responsive Images Using Multiple Backgrounds</h2>

<button data-js="retina">Toggle Retina</button>

<figure>
<div class="crop-height bg-image bg-image-radiocity"><img height="1920" width="2880" class="scale transparent" src="https://s.cdpn.io/23379/photo-radio_city_480x320bw.jpg"></div>

<div class="crop-height bg-image bg-image-camping"><img height="1920" width="2880" class="scale transparent" src="https://s.cdpn.io/23379/photo-camping_480x320bw.jpg"></div>

<div class="crop-height bg-image bg-image-crayons"><img height="1920" width="2880" class="scale transparent" src="https://s.cdpn.io/23379/photo-crayons_480x320bw.jpg"></div>

<div class="crop-height bg-image bg-image-sedona"><img height="1024" width="640" class="scale transparent" src="https://s.cdpn.io/23379/photo-sedona_512x320.jpg"></div>

<figcaption>
Faking "lowsrc": "mobile-first" images all load, then get swapped for high-res. (Empty cache, and reload page to repeat.)
</figcaption>
</figure>

<button data-js="retina">Toggle Retina</button>

</section> <!-- end .page-wrap -->
!

CSS 

              
                .crop-height {
    max-height: 108px;
    height: 108px; /* lt ie8 */
    height: auto;
    overflow: hidden; }

/* base background-image class, specify image below */
.bg-image {
    width: 100%;
    /* prevent scaling past src width (or not) */
    max-width: 1024px;
    /* default background-position */
    background-position: center center;
    /* lt ie 8 */
    -ms-background-position-x: center;
    -ms-background-position-y: center;
    /* scale proportionately */
    background-size: cover;
    /* IE8 workaround - https://louisremi.github.io/background-size-polyfill/ */ }

/* mobile first "lowsrc" - see CSS media queries below for more */

.bg-image-camping {
    background-image: url(https://s.cdpn.io/23379/photo-camping_480x320bw.jpg);
    background-position: center 27%; }

.bg-image-crayons {
    background-image: url(https://s.cdpn.io/23379/photo-crayons_480x320bw.jpg);
    background-position: center 54%; }

.bg-image-radiocity {
  background-image:
    url(https://s.cdpn.io/23379/photo-radio_city_480x320bw.jpg);
    background-position: center 80%; }

.bg-image-sedona {
    background-image: url(https://s.cdpn.io/23379/photo-sedona_512x320.jpg); /* b&w */
    background-position: center 22%; }


/* if you want users to be able to easily save your transparent proxy img,
   be sure to manually declare a height and width on the img element itself
   to match the background-image src size (if the placeholder img is smaller),
   otherwise it won't fill the width of the div so user can't always click on it */

img.scale, object.scale {
  display: block; /* corrects small inline gap at bottom of containing div */
  width: 100% /* corrects obscure Firefox bug */
  max-width: 100%;
  /* just in case, to force correct aspect ratio */
  height: auto !important;
  /*width: auto\9; 
  /* ie8+9 - use modernizr instead of this \9 hack */
  /* lt ie8 */
  -ms-interpolation-mode: bicubic;
    /* force a minimum size if img src size is known */
      /* min-height: 320px; /* max-height of .crop-height */
      /* min-width: 480px; /* consistent with image ratio */
    /* optionally center if img src is not as wide as div */
      /* margin: 0 auto; */ }

.invisible {
    visibility: hidden; }

.transparent {
    /* trigger hasLayout for ie */
    zoom: 1;
    /* 0 value in filters retains layout */
    -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
    filter: alpha(opacity=0);
    opacity: 0; }


/* ==================================================

   MEDIA QUERIES

   .hd class added by jQuery after initial page load
   .multiplebgs class added by modernizr.js

===================================================== */

@media
    /* momma bear, plus any-retina */
    only screen and (min-width : 513px),
    only screen and (-webkit-min-device-pixel-ratio: 1.5),
    only screen and (   min--moz-device-pixel-ratio: 1.5),
    only screen and (     -o-min-device-pixel-ratio: 3/2),
    only screen and (        min-device-pixel-ratio: 1.5),
    only screen and (                min-resolution: 144dpi),
    only screen and (                min-resolution: 1.5dppx) {

        /* no-multiplebgs - medium fallback
           (if you have a mid-sized image, it's probably sufficient here -
           it's unlikely someone with a retina display is on an older browser) */

        .no-multiplebgs .bg-image-sedona.hd,
        .no-js .bg-image-sedona {
            background-image: url(https://s.cdpn.io/23379/photo-sedona_1024x640.jpg); } /* sepia */

        .multiplebgs .bg-image-sedona.hd {
            background-image:
                url(https://s.cdpn.io/23379/photo-sedona_1024x640.jpg),  /* mid-sized on top */
                url(https://s.cdpn.io/23379/photo-sedona_512x320.jpg); } /* "lowsrc" on bottom */

        /* no-multiplebgs high-res fallback */
        .no-multiplebgs .bg-image-camping.hd,
        .no-js .bg-image-camping {
            background-image: url(https://s.cdpn.io/23379/photo-camping2_2880x1920.jpg); }

        /* hi-res retina */
        .multiplebgs .bg-image-camping.hd {
            background-image:
                url(https://s.cdpn.io/23379/photo-camping2_2880x1920.jpg),   /* high-res on top */
                url(https://s.cdpn.io/23379/photo-camping_480x320bw.jpg); } /* "lowsrc" on bottom */

        /* no-multiplebgs high-res fallback */
        .no-multiplebgs .bg-image-crayons.hd,
        .no-js .bg-image-crayons {
            background-image: url(https://s.cdpn.io/23379/photo-crayons2_2880x1920.jpg); }

        /* hi-res retina */
        .multiplebgs .bg-image-crayons.hd {
            background-image:
                url(https://s.cdpn.io/23379/photo-crayons2_2880x1920.jpg),   /* high-res on top */
                url(https://s.cdpn.io/23379/photo-crayons_480x320bw.jpg); } /* "lowsrc" on bottom */

        /* no-multiplebgs high-res fallback */
        .no-multiplebgs .bg-image-radiocity.hd,
        .no-js .bg-image-radiocity {
            background-image: url(https://s.cdpn.io/23379/photo-radio_city2_2880x1920.jpg); }

        /* hi-res retina */
        .multiplebgs .bg-image-radiocity.hd {
          background-image:
            url(https://s.cdpn.io/23379/photo-radio_city2_2880x1920.jpg),  /* high-res on top */
            url(https://s.cdpn.io/23379/photo-radio_city_480x320bw.jpg); } /* "lowsrc" on bottom */

    }


/* moving to larger images after the first "bump" we may choose to separate
   width-based adjustments vs. device-based logic. For example, a larger retina
   device can go directly from "lowsrc" default to high-res with no mid-size image */

@media
    /* poppa bear - smaller here for demo */
    only screen and (min-width : 716px) {

        /* no-multiplebgs - medium fallback above */

        .multiplebgs .bg-image-sedona.hd {
            background-image:
                url(https://s.cdpn.io/23379/[email protected]), /* high-res on top  */
                url(https://s.cdpn.io/23379/photo-sedona_1024x640.jpg),    /* mid-sized in middle */
                url(https://s.cdpn.io/23379/photo-sedona_512x320.jpg); }  /* "lowsrc" on bottom */
    }


@media
    /* larger retina devices, such as iPad, triggers immediately
       so we no need for mid-size image */
    only screen and (min-device-width : 768px) and (-webkit-min-device-pixel-ratio: 1.5),
    only screen and (min-device-width : 768px) and (   min--moz-device-pixel-ratio: 1.5),
    only screen and (min-device-width : 768px) and (     -o-min-device-pixel-ratio: 3/2),
    only screen and (min-device-width : 768px) and (        min-device-pixel-ratio: 1.5),
    only screen and (min-device-width : 768px) and (                min-resolution: 144dpi),
    only screen and (min-device-width : 768px) and (                min-resolution: 1.5dppx) {

        /* no-multiplebgs - medium fallback above */

        .multiplebgs .bg-image-sedona.hd {
            background-image:
                url(https://s.cdpn.io/23379/[email protected]), /* high-res on top  */
                url(https://s.cdpn.io/23379/photo-sedona_512x320.jpg); }   /* "lowsrc" on bottom,  */
    }

/* Presentational CSS Below */

.page-wrap {
  display: block;
  width: 80%;
  max-width: 1280px;
  margin: 0 auto 2em;
  padding: 1.5em 3em 3em;
  background: #fff;
  text-align: left; }

h1 {
  margin: .5em 0 0 0; }

h2 {
  margin: .25em 0 .5em;
  color: #999;
  font-weight: 300; }

figure {
  max-width: 1008px; /* max-width of .bg-image -padding */
  margin: 1.5em 0 .5em; /* change 0 to auto to center */
  padding: 8px 8px 4px 8px;
  background: #eaeaea; }

figcaption {
  margin: .5em 0;
  padding: .125em .5em;
  color: #999;
  text-align: center;
  font-size: 85%;
  font-style: italic; }

code {
  background: lightyellow;
  padding: 1px .25em;
  font-family: Menlo, Monaco, monospace; 
  font-style: normal; }

.nowrap, .nobr, .nobreak {
  white-space: nowrap;
}
!

JS 

              
                  /* waits until everything is loaded, not just DOM is ready */
$(window).load(function() {

    $('.bg-image').toggleClass('hd');

});

/* don't lock class names to functionality, use '[data-js=retina]' */
$('[data-js=retina]').click(function () {
  $('.bg-image').toggleClass('hd');
});
!
999px

Console