Use a static site generator. The parts you touch for authoring posts will end up "included" during the build step, producing files that you don't touch which ea

Use a static site generator. The parts you touch for authoring posts will end up "included" during the build step, producing files that you don't touch which each redundantly contain the common markup. Or with an SPA generator like Gatsby, there wouldn't even be any redundancy (but client side JS is required).