I bet you heard about safetynet on android devices. It is a service that checks if you run a genuine licensed not-modified version of android. If not - app developer can just restrict you access to the app. It is mostly used by banking apps, but there're many examples of not security critical apps utilize this.
Google wants to do the same but for browsers and websites. If you run firefox or modified chrome or use adblocks: youtube, twitter, etc. would be able to detect it and can restrict access to the website.