alt Hacker NewsI think this is the most likely answer.
I'm not defending it, because when I started web development this was one of the first problems I ran into as well -- how the heck do you include a common header.
But the original concept of HTML was standalone documents, not websites with reusable components like headers and footers and navbars.
That being said, I still don't understand why then the frames monstrosity was invented, rather than a basic include. To save on bandwidth or something?
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The original concept of HTML was as an SGML subset, and SGML had this functionality, precisely because it's very handy for document authoring to be able to share common snippets.
A lot of early HTML was about taking the output of a different system such as a mainframe and putting that output into HTML.
Lots of gateways between systems.
Frames were widely abused by early web apps to do dynamic interfaces before XHR was invented/widely supported. The "app" had a bunch of sub-frames with all the links and forms carefully pointing to different frames in the frameset.
A link in a sidebar frame would open a link in the "editor" frame which loaded a page with a normal HTML form. Submitting the form reloaded it in that same frame. Often the form would have multiple submit buttons, one to save edits in progress and another to submit the completed form and move to the next step. The current app state was maintained server side and validation was often handled there save for some basic formatting client side JavaScript could handle.
This setup allowed even the most primitive frame-supporting browsers to use CRUD web apps. IIRC early web frameworks like WebObjects leaned into that model of web app.