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HTML

              
                  <div class="post-inner">
    <h2 class="post"><a href="#">Post title</a></h2>

    <div class="figure left">
      <img src=
      "data:image/gif;base64,R0lGODlhAQABAPAAAN3d3f///yH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==" />

      <div class="title">
        A sample image to go along with the post
      </div>
    </div>

    <div class="meta">
      <span class="comments"><a href="#">Post Comment</a></span> | <span class=
      "tags">Tags: <a href="#">example</a>, <a href="#">rem unit</a>, <a href=
      "#">css</a></span>
    </div>
<p class="instructions">Adjust size of secondary elements to be <a href="#secondary-12px">small</a>, <a href="#secondary-14px">medium</a> or <a href="#secondary-16px">large</a></p>
    <p>This page contains a variety of interface elements. They are not intended to actually work, and instead are here to show how a subset of elements with dimensions specified in rem can change size when &lt;html&gt; element font size is adjusted. Meanwhile, all other content remains unaffected.</p>
    <p>Since this is a technical demo, things will not look pretty, and that's OK.</p>
    
  <div class="meta">
    <span class="date">14 Mar 2013</span> | <span class="author">By Joe Author</span>
  </div>    

    <div class="slideshow">
      <div class="controls">
        <a class="previous" href="#">Previous</a> <a class="next" href="#">Next</a>
        <a class="page active" href="#">1</a> <a class="page" href="#">2</a> <a class=
        "page" href="#">3</a> <a class="page" href="#">4</a> <a class="page" href=
        "#">5</a>
      </div>
      <img src=
      "data:image/gif;base64,R0lGODlhAQABAPAAAKqq/////yH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==" />      
    </div>    
  </div>
              
            
!

CSS

              
                html { font-size: 12px; }
html.secondary-12px { font-size: 12px; }
html.secondary-14px { font-size: 14px; }
html.secondary-16px { font-size: 16px; }

body { font-size: 18px; }
a { text-decoration: none; color: #669; }
.post-inner { font-family: sans-serif; }

.post-inner .meta { font-size: 1rem; }

.post-inner h2 { font-family: sans-serif; font-size: 32px; margin: 0.3em 0;}

.figure { width: 200px; padding: 10px; border: 1px solid #AAA; }
.figure img { width: 100%; }
.figure.left { float: left; margin-right: 10px; margin-bottom: 10px; }
.figure .title { font-size: 1rem; }

.slideshow { margin-bottom: 1em; overflow: hidden; clear: both;}
.slideshow img { width: 100%; height: 200px; }
.slideshow .controls { text-align: center; font-size: 1.25rem; }
.slideshow .previous { float: left; }
.slideshow .next { float: right; }

.slideshow .controls a { border: 1px solid #AAA; padding: 0 0.5rem; }
.slideshow .controls a.active { background: #EEE; }

.instructions { font-size: 1.2em; color: red; }
a[href^="#secondary"] {
  color: white; background: #C00; padding: 0 0.25em;
}
              
            
!

JS

              
                document.addEventListener('DOMContentLoaded', function() { 
  [].forEach.call(document.querySelectorAll('a[href^="#secondary-"]'), function(el, i) {
    el.addEventListener('click', function(ev) {
      ev.preventDefault();
      var docEl = document.documentElement,
          hash = this.getAttribute('href').slice(1);
      if (!hash) return;
      docEl.setAttribute("class", hash);
      docEl.style.display = "none";
      docEl.clientWidth;
      docEl.style.display = "";         
    }, false);
  });
}, false); 
              
            
!
999px

Console