The feature proposal was called HTML Imports [1], created as part of the Web Components effort.
> HTML Imports are a way to include and reuse HTML documents in other HTML documents
There were plans for <template> tag support and everything.
If I remember correctly, Google implemented the proposed spec in Blink but everyone else balked for various reasons. Mozilla was concerned with the complexity of the implementation and its security implications, as well as the overlap with ES6 modules. Without vendor support, the proposal was officially discontinued.
> HTML Imports are a way to include and reuse HTML documents in other HTML documents
There were plans for <template> tag support and everything.
If I remember correctly, Google implemented the proposed spec in Blink but everyone else balked for various reasons. Mozilla was concerned with the complexity of the implementation and its security implications, as well as the overlap with ES6 modules. Without vendor support, the proposal was officially discontinued.
[1] https://www.w3.org/TR/html-imports/