You can write any valid HTML like:
<span id="test" class="kool-class" style="color: red;" fs-test-element="test">This is a cool custom HTML string.</span>
And it will be parsed and rendered.
Additionally, you can disable an entire line from being parsed by adding <!-- fs-richtext-ignore --> at the beginning of it:
<!-- fs-richtext-ignore --> <span id="test" class="kool-class" style="color: red;" fs-test-element="test">This is a cool custom HTML string.</span>