Quarto vignettes used as articles for pkgdown should benefit from the new `renderings` mode

• Markdown syntax
  • https://quarto.org/docs/output-formats/html-themes.html#mode-specific-content
• for Cell outputs
  • https://quarto.org/docs/computations/execution-options.html#cell-renderings

I am still unsure if this is a pkgdown thing to support (by post-processing the HTML output) or if we can help with this from Quattro Vignette engines.

I just know that pkgdown support light / dark: https://pkgdown.r-lib.org/articles/customise.html#light-switch
However, a vignette included in the package should maybe behave differently when renderings is set (probably choosing only the first one)

I think the question is:

• Could we make renderings works for pkgdown article?
• Could there be a fallback mode to use the vignette also as a CRAN package vignette? However, I believe it is fine to say this is only for articles.
• Quarto should still have an opt-out for renderings to only select one mode if not existing already. (e.g. what happens if #| renderings: [light, dark] but no light / dark themes are set?)

This is to discuss and brainstorm

pkgdown context

It is using the Bootstrap color mode (https://getbootstrap.com/docs/5.3/customize/color-modes/).

pkgdown does call quarto_render() but they call a minimal version for the output with no theme
https://github.com/r-lib/pkgdown/blob/f950aed41af49c732ba7884d0806c3641a7b94b3/R/build-quarto-articles.R#L124-L141

quarto_format <- function(pkg) {
  list(
    lang = pkg$lang,
    format = list(
      html = list(
        template = system_file("quarto", "template.html", package = "pkgdown"),
        minimal = TRUE,
        theme = "none",
        `html-math-method` = config_math_rendering(pkg),
        `embed-resources` = FALSE,
        `citations-hover` = TRUE,
        `link-citations` = TRUE,
        `section-divs` = TRUE,
        toc = FALSE # pkgdown generates with js
      )
    )
  )
}

Then they handle the rest.

Start with pkgdown's R/build-quarto-articles.R around lines 124–141 and its quarto_render() call, then read the linked Quarto renderings and Bootstrap color-mode documentation. Determine whether rendering selection belongs in pkgdown or the Quarto vignette engine, including CRAN fallback and opt-out behavior; done requires an agreed scope and implementation path.