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    <p><strong>How</strong></p> <p><a href="http://www.w3.org/TR/CSS2/cascade.html#specificity" rel="nofollow noreferrer">W3C rules for calculating specificity</a>:</p> <blockquote> <ul> <li>count 1 if the declaration is from is a 'style' attribute rather than a rule with a selector, 0 otherwise (= a) (In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, so a=1, b=0, c=0, and d=0.)</li> <li>count the number of ID attributes in the selector (= b)</li> <li>count the number of other attributes and pseudo-classes in the selector (= c)</li> <li>count the number of element names and pseudo-elements in the selector (= d) </li> </ul> <p>The specificity is based only on the form of the selector. In particular, a selector of the form "[id=p33]" is counted as an attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source document's DTD.</p> <p>Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the specificity. Also, when rules have the same specificity, the last one wins.</p> </blockquote> <p><strong>Example</strong></p> <ul> <li><h1>outer span: a=0, b=1, c=0, d=1 --> 101</h1></li> <li>span#inner: a=0, b=1, c=0, d=1 --> 101</li> <li>div#outer span#inner: a=0, b=2, c=0, d=2 --> 202</li> </ul> <p>Try rearranging rules 1 and 3: <a href="http://jsfiddle.net/Wz96w/" rel="nofollow noreferrer">http://jsfiddle.net/Wz96w/</a></p> <p><strong>Why</strong></p> <p>My thought is that #inner does not specify a unique element. While it is only 1 element per page, that ID could be a completely different element on another page.</p> <p>One page:</p> <pre><code>&lt;div id="outer"&gt; &lt;div id="inner"&gt; ... &lt;/div&gt; &lt;/div&gt; </code></pre> <p>Another page:</p> <pre><code>&lt;ul id="outer"&gt; &lt;li&gt;main1 &lt;ul id="inner"&gt; &lt;li&gt;sub1&lt;/li&gt; &lt;li&gt;sub2&lt;/li&gt; &lt;/ul&gt; &lt;/li&gt; &lt;li&gt;main2&lt;/li&gt; &lt;/ul&gt; </code></pre> <p>Although, I would not code it this way. I think it explains why adding the element (<code>ul#outer</code> vs. <code>#outer</code>) is worthy of extra specificity.</p> <p>For the point on descendants, I'm visualizing the DOM for your markup. The rule <code>#outer span</code> has a path length longer than that of <code>#inner</code>. Therefore, in the case, it specifies a more specific subtree, so it should be awarded more specificity (and <code>#outer #inner li</code> should be [is] worth more than <code>#inner li</code>).</p>
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    1. POWhy is the # selector of lesser specificity than anything?
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    1. USTaylor Edmiston
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    1. COCopied and pasted, I apologize for ruining your effort, but the question was about why things are the way that they are, not how to obey them: "Again, I can't emphasize this enough: I'm not asking a question of HOW the #_id_ selector is less than #_id_ element selector, I'm asking why it's made that way."
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    2. COI do appreciate the example, T, but your own sentence should disclaim this example: "Although, I would not code it this way." Shouldn't standards be written for a good coding style? If I attach a style sheet to a page that calls an element by name and tells it how to behave, shouldn't it obey that above any generic rule?
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    3. COI added clarification to express that #inner is not necessarily used on only one element; it may be a different element on other pages. That is part of my hypothesis for why the W3C chose to calculate specificity this way. Interesting question btw.
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