This breaks the social contract of the web. If you publish on the web (over HTTP) using HTML (with or without css+js) then you respect the concept of a User-Agent that renders the content on behalf of the user. If you wish to get around this then create another file format and protocol and application/plugin to do so — one that enforces the DRM for the owner. Don’t take agency away from User-Agent.
Alternatively, the add-ons break the social contract, whereby content creators request payment for their content. If you visit stuff on the web, you respect the concept of a social contract between you and the provide. Don’t try to circumvent their wishes.
If I send a site an http GET request, and that site responds by giving me some data, then that site has given me implied consent to look at the data. If they don't want me to look at it, they shouldn't have given me it.