title: hyperHTML: Custom Boolean Attributes
body: <p>Is it possible to have a custom boolean attribute? In the hyperHTML documentation for Boolean Attributes, it states the following: </p>
  
  <blockquote>
    <p>Just use boolean attributes any time you need them, if it's part of the element's inheritance, it'll always do the right thing.</p>
  </blockquote>
  
  <p>This snippet doesn't work:</p>
  
  <pre><code>this.html`&lt;custom-element myboolean=${this.flag}&gt;&lt;/custom-element&gt;`;
  </code></pre>
  
  <p>If I needed <code>custom</code> to be a boolean attribute, how could I make it so that it's "part of the element's inheritance"?</p>