`hreflang.default`

Default: warn
Plugin: hreflang

Pages with hreflang links also have an x-default link.

Why: x-default names the page to show visitors whose language matches none of the alternates.

When off: alternate links without x-default pass.

<!-- ✗ Fails -->
<link rel="alternate" hreflang="en" href="https://example.com/en/" />
<link rel="alternate" hreflang="de" href="https://example.com/de/" />

<!-- ✓ Passes -->
<link rel="alternate" hreflang="en" href="https://example.com/en/" />
<link rel="alternate" hreflang="de" href="https://example.com/de/" />
<link rel="alternate" hreflang="x-default" href="https://example.com/" />

References

Google: Localized versions of your pages: Google recommends x-default for the fallback page, but does not require it, so the rule warns.