Whitespace removed in CSS properties in unknown @rules

[jansorg]

I'm writing CSS for printing with Prince XML (via hugo). The CSS formats the footnote area, https://www.princexml.com/doc-prince/#footnotes-styling .

Now

div {
  border: 1px solid black;
}

@page {
  @footnotes {
    border: 1px solid black;
  }
}

becomes this (tested with https://go.tacodewolff.nl/minify):

div{border:1px solid #000}@page{@footnotes{border:1pxsolidblack;}}

The whitespace of the border root is removed and thus breaks the printing styles.
It would be great if whitespace within unknown rules could be preserved (if that's the issue here).