I think a particularly bad early mistake of the web, as it exists now, was a category error.

HTTP and HTML were designed to move and display documents. It still does a pretty good job of that, too - when someone points out a well built website, it's typically a small library. Little books of text or images or audio, everything is stateless.

But people didn't want the web to be a library. They wanted to distribute applications. Javascript was born and slowly sent us all to hell.