[Jellby]

Either use a paragraph style with additional top (or bottom) margin, or a specifically styled empty <div> or <p> (or even <hr/>):

<p>... some text.</p>

<div class="break" />

<p>Text after the scene break...</p>

Some readers might discard the empty <div>, so you can also put an &nbsp; inside. Now you can style it as you wish:

div.break {
  height: 2em;
  margin: 0;
  padding: 0;
}

or have a few asterisks inside, or some border...