Indent your code.

Quite simple reasoning to this one, it makes it easier to read. I mean, what would you rather look at when coding? This:

<html>

  <head>

    <title>This is our home page</title>

  </head>

  <body>

    <h1>Hello World!</h1>

    <p>Here's a list of random stuff:</p>

    <ul>

      <li>Alligator</li>

      <li>Radiator</li>

      <li>Flibbetygibbet</li>

    </ul>

  </body>

</html>

Or this:

<html><head><title>This is our home page</title></head><body><h1>
Hello World!</h1><p>Here's a list of random stuff:</p>
<ul><li>Alligator</li><li>Radiator</li><li>Flibbetygibbet</li>
</ul></body></html>