From what I remember, the problem was that it would make it hard for users to distinguish between OS level notifications/vibrations and those coming from a web page (making web page notifications feel more authoritative than they actually are... like "<shake>We detected a virus! download out spamware!<shake>").
Bug 1653318 Comment 5 Edit History
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From what I remember, the problem was that it would make it hard for users to distinguish between OS level notifications/vibrations and those coming from a web page (making web page notifications feel more authoritative than they actually are... like "<shake>We detected a virus! download our spamware!<shake>").