So this is pretty simply, but I feel pretty useful.
To make a horizontal line, use the
tag. With no styling, it will give a result like this:
You can style it just like any other element, using CSS.
<hr style="height:3px;background-color: black;width:90%;" class="line1">
Some credit here, although some of the stuff in that post is deprecated. I believe in HTML5, the best practices are to just use standard CSS to style lines.