Inspect any major website, you'll see boilerplate of <div><div><div><div> and unreadable pile of JavaScript programs your browser need to run to build the website. Sites should be done in a way that is still readable after you disable one element (for example disable CSS function, scripts or HTML specific tag).

Internet is modular, based on stacked protocols. Want to fit Tor between TCP and HTTP? No problem. Web is also like that, build of semi-independent formats, in theory. But in practice devs are using frameworks that assume Chrome, Firefox and Safari are and would ever be the only things existing. Now if you want to develop new browser you not only need to display HTML and add other things later. You need to get all specifications of all standards working right away or sites would spectacularly break.