Not sure if you meant to use <p> as an example because it's well-defined as an optional closing tag.

This is a completely normal usage and some people even author their HTML like this. Granted this is part of the effort to standardize error handling.

That unclosed <p> will auto-close as soon as you start another paragraph or close it's parent element. Likewise this is how I do lists these days:

  <ul>
    <li> One
    <li> Two
  </ul>

The subsequent li's will close the previous and the bounding ul ends the last list item.