how can this be done at the dns level? shouldn't ssl certificates prevent third party content from being shown in the browser?
Well, you get the warning, but as long as HSTS is not active, you can still click on "Accept the risk and continue" …
[EDIT:] Just checked a bit closer, they are using an LetsEncrypt cert for "cuii.telefonica.de", which is obviously the wrong domain, but as I said above, as long as HSTS is not active for "annas-archive.li", you can still bypass via the button.
It does. The browser won't load the content because it detects your connection was tampered with.
They redirect to a different url.
My ISP currently makes them not resolve (with scary sounding domains):