Comment by PaulHoule

Exactly -- that's what is missing in the (at least conventional) HTML and CSS stack.

One of the great ideas from Ted Nelson's

https://en.wikipedia.org/wiki/Project_Xanadu

was transclusion: any document could embed part of some other document. Now Ted saw that being done by a special kind of hyperlink (you're not copying the target but linking to it) and he imagined there was a micropayments system such that I get 10 cents for the document and author of the target gets 1 cent.

In the case of HTML it would be great if you could snip a piece of HTML out of a document together with CSS and associated assets and then incorporate these into another document and have it "just work". I mean, if it was easy to do we wouldn't see people collect and post so many screenshots from X, they could just make an HTML page that embeds the content.

I prototyped a server-side templating system that would break HTML down into a DOM graph and made some progress on the problem of rewriting identifiers such as classes and identifiers in a donor document to address problems similar to the "hygenic macro" problem when it is embedded in a host document. I got it to work in some simple cases but wasn't sure how general I could make it. (In some cases you are going to have to modify the CSS to make the donor work inside the host, for instance, and that probably does involve judgement calls.)

It's quite a miracle that people get CSS to work inside React components, Web components, and stuff like that, given that out of the box CSS doesn't have any mechanism to stop people from using the same classes, not to mention a rather complex mechanism of inheritance, etc.