Here’s my common practice: Every page in the app needs the standard HTML boilerplate with my common stylesheets and JavaScripts. I need a layout that provides these, but makes no assumptions on the actual body of the page. This is my “base” layout. Most – but not all – pages in the app need a common header and footer. This is my “application” (default) layout. I’d like the application layout to reuse the base layout to keep my code DRY.

Really you are not the first to run into such a problem

Quite a lot of programs actually detect if their output goes to a file (e.g. try man | grep -F a and you will not be able to scroll back and forth).

I do think it's a common pattern that should be solved, and I am probably going to try and solve it as a Gem as opposed to simply writing code that we use in our code base