Unexpected (to me) behaviour when embedding a Shiny app in an R Markdown document and styling with CSS:

You can take a look at the screenshots folder if you want a peek without running the code.

In a nutshell, when runtime: shiny is used in an Rmd, div ids get "section-" prepended in the rendered output. Hence, styling is applied correctly to a div named "blah" if the div id is defined as "section-blah" in the css.

I could not find any documentation around this. Is this expected? If so, why?