Remember: The people who would supposed to benefit from a rollout like this are the website operators, not Firefox. The change is happening; I doubt Mozilla is going to reverse course because a couple websites look a little wonky (totally reasonable).
How would they report it? "Hey, your website looks weird. Yeah I'm running Firefox 138 stable and I'm in the 5% experimental group who received the default h1 styling change"?
What is more likely to happen is: A website might get a report, the dev goes to reproduce it, and they're in the control group, so they can't; and because it was only one or two reports, it gets closed.
Firefox also has a built-in "broken website" reporter in the Help menu, which sends a report to Mozilla. The WebCompat team analyzes those reports and can decide to report a Gecko bug if it has a bug, contact the website if they have a bug, or apply a sitepatch. We're also monitoring the broken website reports for regressions from this change in particular.
I understand that a rollout makes reproduction harder, and not everyone will be aware of the change. It's a tradeoff when deciding whether to do a rollout or ride the trains normally.
How would they report it? "Hey, your website looks weird. Yeah I'm running Firefox 138 stable and I'm in the 5% experimental group who received the default h1 styling change"?
What is more likely to happen is: A website might get a report, the dev goes to reproduce it, and they're in the control group, so they can't; and because it was only one or two reports, it gets closed.