R/css.R
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#' Remove comments from CSS
#'
#' Will handle comments with the following form:
#'
#' - \code{/* ... */}
#' - \code{/*! ... */}
#'
#' @noRd
css_remove_comment <- function(css) {
  str_replace_all(css, "/\\*!?(\\*(?!/)|[^\\*])*\\*/", "")
}

#' Inline CSS from various sources
#'
#' - Replace \code{@import()} content in CSS. Assumes that there is just a
#'   single \code{<style>} tag in \code{<head>}.
#' - Extract all sources of CSS and return in named vector with components:
#'
#'   * inline
#'   * external and
#'   * style.
#'
#' @noRd
css_inline <- function(content) {
  # Find first <style> tag in <head>.
  css <- xml_find_first(content, "//head/style")

  if (is.na(css)) {
    log_debug("No embedded <style>.")
  } else {
    # Extract content of <style> tag.
    css_text <- xml_text(css)

    # Iterate over @import statements.
    #
    for (import in unlist(str_match_all(css_text, "@import url\\(.*\\);"))) {
      # Get URL from url().
      url <- import %>% str_remove_all("(^@import url\\([\"']|[\"']\\);$)")
      # Pull content from URL.
      log_debug("Import CSS from {url}.")
      imported <- stri_read_lines(url) %>% paste(collapse = "\n")
      # Replace @import url(); with downloaded content.
      css_text <- sub(import, imported, css_text, fixed = TRUE)
    }

    # Replace content of <style> tag.
    xml_text(css) <- css_text
  }

  c(
    # * Inline CSS in <link> tags.
    inline = xml_find_all(content, "//link[starts-with(@href, 'data:text/css') and @rel='stylesheet']") %>%
      xml_attr("href") %>%
      unlist() %>%
      url_decode() %>%
      str_replace("data:text/css,", ""),
    # * External CSS in <link> tags.
    external = xml_find_all(content, "//link[starts-with(@href, 'http') and @rel='stylesheet']") %>%
      xml_attr("href") %>%
      map(function(href) {
        stri_read_lines(href) %>% paste(collapse = "\n")
      }),
    # * Inline CSS in <style> tags.
    style = xml_find_all(content, "//style") %>%
      xml_text() %>%
      unlist(),
    NULL
  )
}