No x-default
WarningLow priority
Pages with language alternates but no x-default.
Why it matters
Without it nothing says which page to show someone whose language is none of those listed.
How to fix it
Add an x-default alternate, usually the page that lets a visitor choose, or the main language's.
Skitter runs this check on every crawl, and lists each page it finds with the evidence: the header, the tag or the link that gave it away.