CSS ui-monospace polyfill (448 bytes minified, 182 gzipped)

ui-monospace.css

/* These rules are ignored when ui-monospace is supported. */
@font-face { font-family: ui-monospace; src:
	/* MacOS (El Capitan +) */
	local(Menlo-Regular),
	/* Windows (11 +) */
	local(CascadiaCode-Regular),
	/* Windows (Vista +) */
	local(Consolas),
	/* Android */
	local(RobotoMono-Regular),
	/* Gnome */
	local(Cantarell-Regular),
	/* Ubuntu */
	local(UbuntuMono-Regular),
	/* KDE */
	local(OxygenMono-Regular),
	/* Fallback */
	local(Courier)
}

@font-face { font-family: ui-monospace; font-weight: 700; src:
	local(Menlo-Bold),
	local(CascadiaCode-Bold),
	local(Consolas),
	local(RobotoMono-Bold),
	local(Cantarell-Bold),
	local(UbuntuMono-Bold),
	local(OxygenMono-Bold),
	local(CourierBold)
}

ui-monospace.min.css

@font-face{font-family:ui-monospace;src:local(Menlo-Regular),local(CascadiaCode-Regular),local(Consolas),local(RobotoMono-Regular),local(Cantarell-Regular),local(UbuntuMono-Regular),local(OxygenMono-Regular),local(Courier)}@font-face{font-family:ui-monospace;font-weight:700;src:local(Menlo-Bold),local(CascadiaCode-Bold),local(Consolas),local(RobotoMono-Bold),local(Cantarell-Bold),local(UbuntuMono-Bold),local(OxygenMono-Bold),local(CourierBold)}