1 files changed, 79 insertions, 0 deletions
diff --git a/packages/astro/src/assets/fonts/implementations/system-fallbacks-provider.ts b/packages/astro/src/assets/fonts/implementations/system-fallbacks-provider.ts
new file mode 100644
index 000000000..3e2340c78
--- /dev/null
+++ b/packages/astro/src/assets/fonts/implementations/system-fallbacks-provider.ts
@@ -0,0 +1,79 @@
+import type { SystemFallbacksProvider } from '../definitions.js';
+import type { FontFaceMetrics, GenericFallbackName } from '../types.js';
+
+// Extracted from https://raw.githubusercontent.com/seek-oss/capsize/refs/heads/master/packages/metrics/src/entireMetricsCollection.json
+const SYSTEM_METRICS = {
+	'Times New Roman': {
+		ascent: 1825,
+		descent: -443,
+		lineGap: 87,
+		unitsPerEm: 2048,
+		xWidthAvg: 832,
+	},
+	Arial: {
+		ascent: 1854,
+		descent: -434,
+		lineGap: 67,
+		unitsPerEm: 2048,
+		xWidthAvg: 913,
+	},
+	'Courier New': {
+		ascent: 1705,
+		descent: -615,
+		lineGap: 0,
+		unitsPerEm: 2048,
+		xWidthAvg: 1229,
+	},
+	BlinkMacSystemFont: {
+		ascent: 1980,
+		descent: -432,
+		lineGap: 0,
+		unitsPerEm: 2048,
+		xWidthAvg: 853,
+	},
+	'Segoe UI': {
+		ascent: 2210,
+		descent: -514,
+		lineGap: 0,
+		unitsPerEm: 2048,
+		xWidthAvg: 908,
+	},
+	Roboto: {
+		ascent: 1900,
+		descent: -500,
+		lineGap: 0,
+		unitsPerEm: 2048,
+		xWidthAvg: 911,
+	},
+	'Helvetica Neue': {
+		ascent: 952,
+		descent: -213,
+		lineGap: 28,
+		unitsPerEm: 1000,
+		xWidthAvg: 450,
+	},
+} satisfies Record<string, FontFaceMetrics>;
+
+type FallbackName = keyof typeof SYSTEM_METRICS;
+
+// Source: https://github.com/nuxt/fonts/blob/3a3eb6dfecc472242b3011b25f3fcbae237d0acc/src/module.ts#L55-L75
+export const DEFAULT_FALLBACKS = {
+	serif: ['Times New Roman'],
+	'sans-serif': ['Arial'],
+	monospace: ['Courier New'],
+	'system-ui': ['BlinkMacSystemFont', 'Segoe UI', 'Roboto', 'Helvetica Neue', 'Arial'],
+	'ui-serif': ['Times New Roman'],
+	'ui-sans-serif': ['Arial'],
+	'ui-monospace': ['Courier New'],
+} satisfies Partial<Record<GenericFallbackName, Array<FallbackName>>>;
+
+export function createSystemFallbacksProvider(): SystemFallbacksProvider {
+	return {
+		getLocalFonts(fallback) {
+			return DEFAULT_FALLBACKS[fallback as keyof typeof DEFAULT_FALLBACKS] ?? null;
+		},
+		getMetricsForLocalFont(family) {
+			return SYSTEM_METRICS[family as FallbackName];
+		},
+	};
+}