title: When setting CSS font-size in pixels, does the value refer to the width or height of a letter?
body: <p>Does the pixel value of a font-size refer to the height of the font or the width of the font?</p>
  
  <p>CSS:</p>
  
  <pre><code>.sixteen {
    font-size: 16px;
  }
  </code></pre>
  
  <p>HTML:</p>
  
  <pre><code>&lt;span class='sixteen'&gt;a&lt;/span&gt;
  </code></pre>
  
  <p>Will the "a" be 16 pixels tall or 16 pixels wide, or is that like a max-width/height on both so that the entire character fits in a 16px box?</p>
  
  <p>All the documentation I have seen for this has been ambiguous on the matter.</p>