As others said, all that could be done in the 90s modulo the bandwidth (specifically important for Khan Academy). But we don't even have to go back that far to get broken sites. IE 8 (released in 2009), forced on many corporate workers, is unable to render many "modern" sites. Some specific examples that I've encountered: Lifehacker & kin - multiple content blocks overlapping, rendering the main content unreadable; NPR - home page totally broken, can't get to different news categories; GigaOM - ARTICLES ARE PRESENTED IN ALL CAPS FOR NO GOOD REASON, AS IF THE AUTHOR WANTED TO SHOUT AT ME FOR USING SUCH AN OLD BROWSER.
And your 4) we could do that in the 90s as well. Email isn't a 21st century invention and doesn't depend on web technologies. Same with instant messengers. Skype - for video, bandwidth is the thing the 90s lacked to support this system. WhatsApp - widespread mobile devices was necessary for this, 90s lacked that. And none of them, other than Facebook, is wholly dependent on web stuffs, they're internet technologies and platforms.
And your 4) we could do that in the 90s as well. Email isn't a 21st century invention and doesn't depend on web technologies. Same with instant messengers. Skype - for video, bandwidth is the thing the 90s lacked to support this system. WhatsApp - widespread mobile devices was necessary for this, 90s lacked that. And none of them, other than Facebook, is wholly dependent on web stuffs, they're internet technologies and platforms.