Code
Code is a powerful block - but with great power comes great responsibility.
With Code, you as a teacher can embed your own HTML, CSS, and JavaScript into an external page. You can also use the Full page checkbox to choose whether to show that code underneath a header, or to be on a completely otherwise-unstyled page.
For example, imagine you put in the HTML field:
You will see this upon opening the element:
If you have Full page marked, you will see this:
As you can tell, this is quite a bit of freedom. But we trust you - and our old learning platform, Moodle, always supported this in summary blocks (but in a much worse way).