title: Do custom markup tags placed in an HTML document affect the flow and is it possible to parse them with JS?
body: <p>Would it be possible to do the following</p>
  
  <pre><code>&lt;body&gt;
      &lt;main&gt;
          &lt;!-- CONTENT --&gt;
      &lt;/main&gt;
      &lt;foot&gt;
          &lt;!-- FOOTER CONTENT --&gt;
      &lt;/foot&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>if I then wrote some JavaScript that did something along the lines of the following. Please note that I don't want you to write the actual code that goes here. This is just a mockup of the core functionality. </p>
  
  <pre><code>for(elem inside main){
      elem.makeItBig();
  }
  for(elem inside foot){
      if(elem is img){
          elem.makeItSmall();
      }
  }
  </code></pre>
  
  <p>I am aware of this post <a href="https://stackoverflow.com/questions/2802687/is-there-a-way-to-create-your-own-html-tag-in-html5">Is there a way to create your own html tag in HTML5?</a>. But I don't really want to create tags to style them but rather to provide identifying attributes to the DOM which I can hook into using JavaScript. Imagine something kind of like a class, but used in a way that you can stitch lots of PHP generated parts together using these tags. </p>
  
  <p>If you use a made up tag in HTML is it ignored by your browser or will it throw an error. </p>