If you don't want to use CSS you can just use:
Where you want a blank line.
But I tend to create a special CSS class that has an extra-large "margin-top" attribute and assign that class to paragraphs when I want a space (scenebreak or whatever).
Code:
<p class="gap">Blah, blah, blah.</p>
And CSS that looks like:
Code:
.gap { margin-bottom:0; margin-right: 0; margin-top: 1em; margin-left:0; text-indent: 1.2em; }