CodePen

HTML

            
              <!-- Entire Viewport -->
<h1>Entire Viewport</h1>
<div class="viewport">
	<p>Width <code>%</code></p>
	<div class="box-width percentage">100%</div>
	<p>Width  <code>px</code></p>
	<div class="box-width pixel">200px</div>
	<p>Width  <code>em</code> <small>(based on 16px computed font-size)</small></p>
	<div class="box-width em">20em</div>
	<p>Width  <code>rem</code> <small>(based on 16px computed font-size of root element)</small></p>
	<div class="box-width em">20rem</div>
</div>

<!-- Container 480px -->
<h1>Container 480px</h1>
<div class="container">
	<p>Width <code>%</code></p>
	<div class="box-width percentage-half">50%</div>
	<div class="box-width percentage-full">100%</div>
	<p>Width  <code>px</code></p>
	<div class="box-width pixel">200px</div>
	<p>Width  <code>em</code> <small>(based on 16px computed font-size)</small></p>
	<div class="box-width em">20em</div>
	<p>Width  <code>rem</code> <small>(based on 16px computed font-size of root element)</small></p>
	<div class="box-width em">20rem</div>
</div>

<!-- Absolute Position -->
<h1>Absolute Position</h1>
<div class="absolute-position">
	<div class="box-width no-width">Based on content if width is not defined</div>
  <div class="box-width rem"></div>
  <div class="box-width em"></div>
</div>

<!-- Table Rows -->
<h1>Table Rows :(</h1>
<table>
	<tr class="box-width row-percent">
		<td>%</td>
	</tr>
	<tr class="box-width row-em">
		<td>em</td>
	</tr>
</table>
            
          
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via HTML Inspector

CSS

            
              // ========================================
// Globals
// ========================================

html {
	background: #292a2b;
	color: #FFF;
}

.box-width {
	height: 50px;
	background: #e78629;
}


// ========================================
// Demo
// ========================================

.viewport {
	outline: 1px solid #ed862a;
	.box-width.percentage {
		width: 100%; // %
	}
  .box-width.pixel {
		width: 200px; // px
	}
	.box-width.em {
		width: 20em; // em
	}
	.box-width.em {
		width: 20rem; // rem
	}
}


// ========================================
// Container 480px
// ========================================

.container {
	outline: 1px solid #ed862a;
	width: 500px;
	.box-width.percentage-half {
		width: 50%; // half %
	}
	.box-width.percentage-full {
		width: 100%; // full %
	}
	.box-width.pixel {
		width: 200px; // px
	}
	.box-width.em {
		width: 20em; // em
	}
	.box-width.em {
		width: 20rem; // rem
	}
}


// ========================================
// Table Rows and Row Groups
// ========================================

.absolute-position {
  position: relative;
  .box-width.no-width { // width is calculated based on content
    outline: 1px solid black;
    position: absolute;
    top: 0;
    right: 0;
  }
}


// ========================================
// Table Rows and Row Groups
// ========================================

.box-width.row-percent {
	width: 50%;
}

.box-width.row-em {
	width: 20em;
}
            
          
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JS

            
              
            
          
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