Earlier quoted context omitted.
user extensions affecting how the website works That's really the point of having them, isn't it? To make the site work the way the user wants. In fact I'd say what it will actually achieve is less visitors to your site... they're just going to leave and go somewhere else. I classify this along with the anti-right-click, disabling select/copy/paste, changing the status bar, resizing windows, and other general "DO NOT…
What do you do if you create a website which modifies the dom, but find out what your users have extension XYZ which completely breaks are our assumptions about what the dom is? For example you just received a server-rendered page which should have "ul/li/span.Foo", but find out that the span is a link now instead, so your JS breaks down. All of the things you mentioned are websites trying to break your expected beha…
I get to choose how this client-side Javascript runs because I get to choose how every kind of software runs on my own computer.