3.16 improvement - Consider using a different event to trigger the beacon script

[MathieuLamiot]

Context It has been discussed here and here that other events might be used instead of DOMContentLoaded to trigger the beacon script. DOMContentLoaded might be too early as we might not have all the needed information about images. The following alternatives have been suggested: load, window.onload.

Expected behavior At the time this issue is written, the expected behavior seems already working. If we find cases where it does not work, we'll add it here.

Acceptance Criteria At the time this issue is written, the expected behavior seems already working. If we find cases where it does not work, we'll add it here.