Instead of using <hr>, you can one of the border of the enclosing block and display it as a horizontal line.
Here is a sample code:
The HTML:
<div class="title_block">
<h3>This is a header.</h3>
</div>
<p>Here is some sample paragraph text.<br>
This demonstrates that a horizontal line goes between the title and the paragraph.</p>
The CSS:
.title_block {
border-bottom: 1px solid #ddd;
padding-bottom: 5px;
margin-bottom: 5px;
}