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CSS CSS Basics (2014) Understanding Values and Units Em and Rem Units

I don't understand what 'rem' is

it says set the value of h2 to equivalent of 53px in with em

style.css
/* Complete the challenge by writing CSS below */

header {
  font-size: 1.8em;
}

.title {
  font-size: 1.625rem;
}

h1 {
  font-size: 5.625rem;
}

h2 {
font-size: 1.5em;
}
index.html
<!DOCTYPE html>
<html>
  <head>
    <title>Lake Tahoe</title>
    <link rel="stylesheet" href="page.css">
    <link rel="stylesheet" href="style.css">
  </head>
  <body>
    <header>
      <span class="title">Journey through the Sierra Nevada Mountains</span>
      <h1>Lake Tahoe, California</h1>
    </header>
    <div class="main-content">
      <p>
        Lake Tahoe is one of the most breathtaking attractions located in California. It's home to a number of ski resorts, summer outdoor recreation, and tourist attractions. Snow and skiing are a significant part of the area's reputation.
      </p>
      <a href="#">Find out more</a>
      <h2>Check out all the Wildlife</h2>
      <p>
        As spawning season approaches, the fish acquire a humpback and protuberant jaw. After spawning, they die and their carcasses provide a feast for gatherings of mink, bears, and Bald eagles.
      </p>
      <a href="#">See the Wildlife</a>
      <h3>From Tents to Resorts</h3>
      <p>
        Lake Tahoe is full of wonderful places to stay. You have the ability to sleep in the outdoors in a tent, or relax like a king at a five star resort. Here are our top three resorts:
      </p>
    </div>
    <footer>
      <p>All rights reserved to the state of <a href="#">California</a>.</p>
      <a href="#top">Back to top &raquo;</a>
    </footer>
  </body>
</html>

1 Answer

Justin Black
Justin Black
24,266 Points

CSS3 introduces a few new units, including the rem unit, which stands for "root em". If this hasn't put you to sleep yet, then let's look at how rem works.

The em unit is relative to the font-size of the parent, which causes the compounding issue. The rem unit is relative to the root—or the html—element. That means that we can define a single font size on the html element and define all rem units to be a percentage of that.

html { font-size: 62.5%; } 
body { font-size: 1.4rem; } /* =14px */
h1   { font-size: 2.4rem; } /* =24px */

I'm defining a base font-size of 62.5% to have the convenience of sizing rems in a way that is similar to using px.

But what pitiful browser support do we have to worry about?

You might be surprised to find that browser support is surprisingly decent: Safari 5, Chrome, Firefox 3.6+, and even Internet Explorer 9 have support for this. The nice part is that IE9 supports resizing text when defined using rems. (Alas, poor Opera (up to 11.10, at least) hasn't implemented rem units yet.)

What do we do for browsers that don't support rem units? We can specify the fall-back using px, if you don't mind users of older versions of Internet Explorer still being unable to resize the text (well, there's still page zoom in IE7 and IE8). To do so, we specify the font-size using px units first and then define it again using rem units.

html { font-size: 62.5%; } 
body { font-size: 14px; font-size: 1.4rem; } /* =14px */
h1   { font-size: 24px; font-size: 2.4rem; } /* =24px */

And voila, we now have consistent and predictable sizing in all browsers, and resizable text in the current versions of all major browsers.

Thanks for taking your time