> Are only gui based webbrowsers with large market share the only legal way to access multimedia content?
I believe they are the only supported way to access content on YouTube. As for whether the unofficial ways to access YouTube are legal, that depends on whether they circumvent a “technological measure that effectively controls access” [1] to copyrighted works. That’s not so clear, but YouTube does obfuscate its code to make the task of tools like youtube-dl harder.