They are only implemented in Firefox.

Spec change: https://github.com/whatwg/html/pull/6343
Test change: https://github.com/web-platform-tests/wpt/pull/27395

Related Bugzilla bugs:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1392710 (adds use counters)
• https://bugzilla.mozilla.org/show_bug.cgi?id=880425 (seems like this bug would obviate that cleanup)

The telemetry shows these events are limited in use and probably just abused or just as custom events perhaps?

onbounce - 0.02% https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2021-01-28&include_spill=0&keys=__none__!__none__!__none__&max_channel_version=nightly%252F87&measure=USE_COUNTER2_ONBOUNCE_PAGE&min_channel_version=null&processType=*&product=Firefox&sanitize=1&sort_by_value=0&sort_keys=submissions&start_date=2021-01-25&table=0&trim=1&use_submission_date=0
onstart - 0.10% https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2021-01-28&include_spill=0&keys=__none__!__none__!__none__&max_channel_version=nightly%252F87&measure=USE_COUNTER2_ONSTART_PAGE&min_channel_version=null&processType=*&product=Firefox&sanitize=1&sort_by_value=0&sort_keys=submissions&start_date=2021-01-25&table=0&trim=1&use_submission_date=0
onfinish - 0.11% https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2021-01-28&include_spill=0&keys=__none__!__none__!__none__&max_channel_version=nightly%252F87&measure=USE_COUNTER2_ONFINISH_PAGE&min_channel_version=null&processType=*&product=Firefox&sanitize=1&sort_by_value=0&sort_keys=submissions&start_date=2021-01-25&table=0&trim=1&use_submission_date=0

Anyway as no others implement I think it's been safe to remove for a long time. I submitted a PoC patch, I've not verified that I've got rid of it all.

Those are surprisingly high numbers.

The bug assignee didn't login in Bugzilla in the last 7 months.
:hsinyi, could you have a look please?
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