render: real-world CSS rendering parity

Colors: oklch()/oklab() and color-mix(); linear-gradient backgrounds.
Grid: named-line placement, repeat() expansion, drop whitespace-only
grid items; upgrade taffy 0.7 -> 0.12 so aspect-ratio images in
auto-height rows no longer collapse to slivers.
CSS: @layer, nesting, @property, :is()/:where(), clamp(), @media not all;
source/track/param/area and svg desc/metadata as display:none.
Images: webp + intrinsic sizing, srcset/picture, browser UA + Accept for
CDN fetches, grey placeholder on failure; width/height HTML attributes
rank below author CSS with aspect-ratio preserved for auto axes.

crates/obscura-js/js/bootstrap.js

@@ -4527,8 +4527,12 @@ if (typeof TextDecoder === 'undefined') {
 globalThis.matchMedia = _markNative(function matchMedia(q) {
   var s = (q || '').toLowerCase().replace(/\s+/g, '');
   var matches = false;
-  if (s.includes('prefers-color-scheme:light')) matches = false;
-  else if (s.includes('prefers-color-scheme:dark')) matches = true;
+  // Match headless Chromium's default: light color scheme. Reporting dark
+  // here made every JS-driven site (mdBook, docs, apps) switch itself into a
+  // dark theme, rendering nothing like a normal headless-Chrome screenshot.
+  if (s.includes('prefers-color-scheme:dark')) matches = false;
+  else if (s.includes('prefers-color-scheme:light')) matches = true;
+  else if (s.includes('prefers-color-scheme:no-preference')) matches = false;
   else if (s.includes('prefers-reduced-motion:no-preference')) matches = true;
   else if (s.includes('prefers-reduced-motion:reduce')) matches = false;
   else if (s.includes('any-pointer:fine')) matches = true;