fix(font): not emitting 400 as default value when font weight is omitted

[NriotHrreion]

Closes #2793

Overview

The next/font/local shim emits font-weight: 400 when no weight was specified. For variable fonts with a wght axis, this descriptor clamps the font face to its regular 400 instance, preventing heavier weights from rendering correctly even when elements use font-weight: 600 or 700.

This aligns vinext with Next.js and the CSS Fonts specification by allowing the descriptor to retain its default auto behavior when omitted, while preserving explicit single weights, weight ranges, and source-level overrides. The fix applies consistently to both development and production output.

What changed

The original behavior sets font weight to 400 if weight is not provided

vinext/packages/vinext/src/shims/font-local.ts

Line 120 in d65ff09

const weight = sanitizeFontDescriptorValue(src.weight ?? options.weight ?? "400") ?? "400";

vinext/packages/vinext/src/shims/font-local.ts

Lines 132 to 138 in d65ff09

	rules.push(`@font-face {
	font-family: '${escapeCSSString(family)}';
	src: url('${escapeCSSString(src.path)}') format('${format}');
	font-weight: ${weight};
	font-style: ${style};
	font-display: ${display};
	}`);

The new behavior doesn't set font weight if weight is not provided, leaving it as auto

rules.push(`@font-face {
  font-family: '${escapeCSSString(family)}';
  src: url('${escapeCSSString(src.path)}') format('${format}');
- font-weight: ${weight};
- font-style: ${style};
+ ${weight === undefined ? "" : `  font-weight: ${weight};\n`}  font-style: ${style};
  font-display: ${display};
}`);

Testing

Corresponding tests are added or updated.

• pnpm test tests/app-router-font-google-prod.test.ts tests/font-local-transform.test.ts tests/pages-router-font-google-prod.test.ts

Notes

I ran into this issue when migrating my project to vinext. Applying the fix is quite important to me. Thanks for reviewing!