The HTML block gives you great power, which, as Spiderman will attest, comes with great responsibility.
In a content block, if you add some style codes to an element, Concrete5 will remove anything too advanced - to make sure the complex editor keeps working happily. But in the html block, you can close a dive you did not open, or open a div you do not close. That can mess up your page so much, all you can do is delete it from the sitemap and start over.
You can add iframes here, which can be useful - and for any element you can create complex inline styles - which is okay for development and testing, but really all styles should be in an external stylesheet.
Save when you are finished, and move on to the next block