If the point was simply to stream text on to a screen for someone to read, we wouldn't need CSS to begin with.
> and no one really needed to align things to the bottom of the page.
Documents have footers. Most company webpages have useful information in their footers. I think it is reasonable that a web formatting spec offers the tools to properly format these footers. And formatting a footer is not as simple as just "adding text at the end of the content". Yet, it is also not very difficult to specify how it should properly work.
This is just another example to illustrate that the CSS standardization process has been hopelessly lagging for very straightforward use-cases.
Should a footer align to the bottom of the content or the viewport or should it be sticky? Should I be able to continue content after the footer? What happens if I have a second footer element? Should the footer create a new section in the document outline? Should it display inline or as a block element? What should be the default behaviour if I print a page with a footer? Is the footer of a document the same as a footer on a website, and is that the same as the footer on a web app?
These are easy questions I can think of in 30 seconds. No doubt there are some much more complex issues too. The idea that defining a footer element for HTML is easy is nonsense.
As it is, HTML5 has had a <footer> tag for years, and it doesn't do what a document footer does. HTML5 added <header>, <main>, <section>, <aside> and more. None of them do what TeX does.
The reason those things are in TeX is because Knuth was replacing typesetting for his book. He added what he needed. Browser developers did the same thing for webpages, and no one really needed to align things to the bottom of the page.