Fix <embed> not triggering onLoad

[cherniavskii]

Fixes #15613

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[philipp-spiess]

Thank you for your PR @cherniavskii!

Implementation wise this looks solid to me. Do you mind adding a test as well? You can base it off this test case.

[cherniavskii]

@philipp-spiess sure, I'll update this PR later

[cherniavskii]

@philipp-spiess I've added a unit test, which fails without applying patch on this branch

[philipp-spiess]

This looks great! Thank you.

I can merge later if nobody objects 🙂

[nhunzaker]

Awesome. This looks good to me.

[philipp-spiess]

Thanks again 🙌