Minification flattens Prince's @prince-overlay inside @page, breaking printable output

Summary

Since output minification was added (25.0.0; 24.0.0 has no lib/minify.js), an inline <style> containing Prince's @prince-overlay page-region rule is rewritten by clean-css into a plain @page rule. The overlay's declarations — including font-size: 200pt — end up applied to the page itself, which pushes the @top-left / @top-right header image out of the top margin and over the body text, and the overlay is lost.

css/print.css ships this exact rule commented out with "Uncomment when producing WIP versions of final standards", so it's the documented way to get a DRAFT watermark in --printable builds — and it can't work with minification on.

Reproduction

Spec <style> (copied from the commented block in css/print.css):

@page {
  @prince-overlay {
    color: rgba(0,0,0,0.15);
    content: "DRAFT";
    font-family: Arial;
    font-weight: bolder;
    font-size: 200pt;
    transform: rotate(-60deg);
  }
}

ecmarkup spec.html --printable --assets external out/index.html emits:

@page{content:"DRAFT";font-family:Arial;font-weight:bolder;font-size:200pt;transform:rotate(-60deg)}

Standalone, with the clean-css that ecmarkup bundles (5.3.3), at both level: 1 and level: 2:

const CleanCSS = require('clean-css');
new CleanCSS({ level: 2 }).minify(
  '@page { @prince-overlay { content: "DRAFT"; font-size: 200pt } }'
).styles
// => '@page{content:"DRAFT";font-size:200pt}'

Standard page-margin boxes (@top-left etc.) are preserved — it's the vendor at-rule that gets flattened.

Effect in Prince (20240705 for Books, via js/print.js)

Measured on an A4 body page (margin-top: 28mm):

No DRAFT overlay in the minified case.

Real-world instance: Ecma-TC54/ECMA-xxx-TEA (ecmarkup 25.0.0) — every page of the CI PDF had the header over the text; working around it with --no-minify on the printable build.

Suggested fixes (any of)

• Skip CSS minification (or minification entirely) when --printable is set — that HTML only exists to be fed to Prince, so there's nothing to gain.
• Configure clean-css to leave unknown at-rules alone (e.g. via level: { 1: { specialComments... } } / compatibility, or pre-extract @page blocks and reinsert them untouched).
• At minimum, note in css/print.css next to the @prince-overlay block that --no-minify is required.

Start with lib/minify.js and the printable path in js/print.js, then compare the minified output with the CSS block documented in css/print.css. Reproduce the clean-css transformation from the issue and verify that a printable build preserves @prince-overlay, page-margin boxes, and the expected header and body layout.