Comment by dfabulich

Declarative partial updating "sets the stage for client-side includes." https://github.com/WICG/declarative-partial-updates

The linked article suggests a potential syntax:

  <template for="footer" patchsrc="/partials/footer.html">

That would transclude the content of /partials/footer.html in your HTML.

But the road ahead for this is still quite bumpy. Here's a good video from a year ago, talking through the obstacles. https://www.youtube.com/watch?v=t0NBcve0enY