[1718627440]

If they insist on only using vanilla HTML then the problem is unsolved to this day. I think it is actually less solved now, since back then HTML was an SGML application, so you could supply another DTD and have macro-expansion on the client.

[mixmastamyk]

Object tag can do it. iframe also with limitations.

[1718627440]

Does it really? I think, this makes you have a wrapper and I am not sure if you can get rid of all issues with "display: contents". Also you are already in the body, so you can't change the head, which makes it useless for the most idiomatic usecase for that feature.

[mixmastamyk]

Gets you header, footer, components. Most of head would be nice but you typically want a custom title for example.