v151 · performance · prerendering
Feature Detection — form_submission
Check whether the browser supports speculation rules, whether form_submission is accepted in a rule set, and inspect any active prerendering state.
The form_submission field (Chrome 151) extends speculation rules to support prerendering form submissions — not just link navigations. There is no client-side API to verify whether the browser accepts the field, so check 4 reports it as indeterminate. The other checks probe real signals: document.prerendering and PerformanceObserver.supportedEntryTypes.
references
implementation reference
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