Wasn't this in HTTP/1.1 (1997), where you could send chunks to anything that could have a target attribute? (As multipart HTML relied on http chunks, which suffered poor support by the Windows http stack, this quickly vanished and in the 2000s support was stripped from browsers. I think, Chrome was the first to do so. Personal note: I once wrote a chat server, which relied on this technology – and it worked pretty well, outside IE.)
Another way this had been once supported was in Netscape 4.0's layers (1997), which could have a `src` attribute and worked just as you would expect. (However, support for this vanished with the first iterations, about the same time JS-styles were switched to CSS. It had been definitely a feature in the NS 4.0 prerelease versions. Especially the capability to re-link the `src` attribute by JS or by a link target was nice.) NS 4 also featured extended HTML entities, which could render JS expressions in place and also provided a mechanism for conditional comments – so, had we followed that route, HTML would have been already a templating language.