Invalid <track kind> values behave like "metadata", not "subtitles"

Category: Multimedia
Type: New or changed feature
Status: Enabled by default (Chrome 52)
Intent stage: None

Summary

Invalid values for <track kind> are currently treated as "subtitles". This means that any new values will be treated as subtitles by old UAs. Instead use "metadata" as the "invalid value default", to avoid UAs showing <track>s with unrecognized values.

Standards & signals

Specification: https://html.spec.whatwg.org/multipage/embedded-content.html#attr-track-kind
Firefox: Shipped/Shipping
Safari: No signal
Web developers: No signals
Tracking bug: https://bugs.chromium.org/p/chromium/issues/detail?id=608772

Docs: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/track

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