Thanks for sharing Per!
We might be lucky that my company had entirely switched to using Chrome two years ago, so we didn't suffer that issue during migration.
Our biggest challenge was "How to add React/ Svelte to a legacy PHP project incrementally?"
We tried to add php-v8 to run JavaScript at the server-side, but we gave up due to some compatibility problems.
Our last approach was to create SPA for each route, and luckily SvelteKit allows me to build my entry point as a PHP file so that I can copy my production build to the legacy PHP project, and it works like a charm.
Svelte and Sveltekit are great in many ways. However, if you need to support legacy platforms with Sveltekit, you're in trouble, due to this issue: github sveltejs/kit/issues/12. The package @vitejs/plugin-legacy is no longer supported. Maybe it will be fixed some time in the future, but judging from the conversation in the ticket, it might take a while. Just saying, so you don't start a project, like I did, just to eventually find out you can't use the framework...