Because as with many, many, many APIs that Chrome ships it's not a spec:

- there's a giant banner on the left saying "unofficial"

- The "Status of this document" section states the following:

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This document is a draft of a potential specification. It has no official standing of any kind and does not represent the support or consensus of any standards organization.

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But just because it was shipped in Chrome and has some napkin scribbles resembling an official spec, people will both claim it's a spec and expect everyone to implement it.