title: Markdown heading type priority
body: <h1>Here is a title</h1>
  
  <h1>Here is a title in another format</h1>
  
  <h2>Here is a heading</h2>
  
  <h2>Here is a heading in another format</h2>
  
  <h1># What is this?</h1>
  
  <h2>## And what is this?</h2>
  
  <p>Source...</p>
  
  <pre><code>Here is a title
  ===
  
  # Here is a title in another format
  
  Here is a heading
  ---
  
  ## Here is a heading in another format
  
  # What is this?
  ===
  
  ## And what is this?
  ---
  </code></pre>
  
  <p>I can see how markdown is rendered, but I want to know if there is a rule in the spec that gives priority to one form of heading over the other, and how to handle text using both forms at the same time.</p>