Connection not fully secure on Brave Browser

Hello! I’ve just recently started on the course and though I should drop a question here in the hopes someone can explain to me what is going on!

From section 4 - Steganography, Nathan offers a website to check out:

When I go here with the Brave browser I get ‘Not secure’ status next to the URL. When i interrogate this it tells me the certificate is valid. So my question is why does Brave think it is not secure?

I checked Braves help page on connection status but it really didn’t illuminate anything.

If I use Firefox to browse the same URL i get a grey padlock!

(╯°□°)╯︵ ┻━┻

Both Chrome (take any chromium-based browser, Brave or Edge too) and Firefox shows that some parts of this site are not secure. This is done because some parts of the website do not use private communication and someone can do MitM attack. It basically means the TLS cert is not properly implemented for the full website