Vibration API - how to handle iframes

Hi, I'm implementing the Vibration API in Blink and Chromium and noticed
that the expected behavior is not explicitly specified for iframes.

Should the patterns be additive - vibration only stops when all patterns in
all iframes have fully run. This is kind of like playing sounds from
multiple sources at the same time, they can overlap.

Or should we be more conservative and stop vibration when any pattern in
any iframe ends, or navigator.vibrate(0) is called anywhere?

Regards,

Michael van Ouwerkerk