Just try validating the site again.
Check this out for example (The first validation error)
Line 84, Column 312: No element strong to close.
…ou click on the question the answer will drop down below it.
Here is your HTML
Like the Tab Box, this is another way of keeping your content cleanly organized. The Accordion is often used as a vertical navigation menu. It’s a different way of doing dropdown menus. This is also often used for Q&As as well. If you click on the question the answer will drop down below it.
You have a closing strong tag, but no opening tag. If you remove that, or add an opening tag, it will fix that error.