title: Select elements with empty or not-specified attribute
body: <p>Take the following, schematic html-code:</p>
  
  <pre><code>&lt;div&gt;
    &lt;span id='1' cust-attr='' /&gt;
    &lt;span id='2' /&gt;
    &lt;span id='3' cust-attr='Foo' /&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Now I am looking for a selector finding all <code>span</code>s which either do not have an attribute "cust-attr" or whose "cust-attr" value is empty.<br>
  In this case, this would be the 1 and 2.</p>
  
  <p>I tried the following selectors with the following results:</p>
  
  <ol>
  <li><code>span[cust-attr!=]</code> selects 2 and 3</li>
  <li><code>span[cust-attr='']</code> only selects 1</li>
  <li><code>span:not([cust-attr])</code> selects 2</li>
  <li><code>span(:not([cust-attr]),[cust-attr=''])</code> selects all three</li>
  <li><code>span([cust-attr=''],:not([cust-attr]))</code> selects 1</li>
  </ol>
  
  <p>However, I did not find one selecting only 1 and 2.<br>
  Do you know a possibility?</p>
  
  <p>Note that I want to avoid:</p>
  
  <pre><code>span:not([cust-attr]),span[cust-attr='']
  </code></pre>
  
  <p>as "span" is in reality a more complex expression. </p>