A factor contributing to the legibility of fonts at different sizes is the relationship between &lt;span class=&quot;plainlinks&quot;&gt;[http://www.thepiggybackrider.com/ &lt;span style=&quot;color:black;font-weight:normal; text-decoration:none!important; background:none!important; text-decoration:none;/*CITATION*/&quot;&gt;child carrier&lt;/span&gt;]&lt;/span&gt; <span class="plainlinks">[http://www.thepiggybackrider.com/ <span style="color:black;font-weight:normal; text-decoration:none!important; background:none!important; text-decoration:none;/*CITATION*/">kid carrier</span>]</span> the height of the 'font-size' and the height of the font's x-height (the height of its lowercase letter 'x'.) The ratio between these two factors ('font-size' divided by x-height) is called a font's &amp;quot;"aspect value.&amp;quot; " If an aspect value ratio is large for a font, it is more likely to be legible at a given font-size than a font with a smaller aspect value.

The '''font-size-adjust''' property allows authors to specify the &amp;quot;"aspect value&amp;quot; " that they wish to maintain. It becomes helpful when a specified font is unavailable and the system needs hints to determine the most suitable substitute.

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