CommonMark’s ability to set classes and identifiers is amazing. Hugo (which I use to generate this notes) is using it. They work like so, in CommonMark markdown:
Hello world.
{.foo .bar #foh}
Which, in turn, will render and HTML code like this:
<p class="foo bar" id="foh">Hello world.</p>
Now all you have to do is define the class and id on your CSS, or whatever. Neat, right?