> #1 priority is reliability

Yeah, mine is too. (In fact, I think we're approaching a crisis of reliability out there, if we're not there already.) That's why I went with Elixir, and it's paid off so far. The only time the site's been down in years is due to my hosting provider, not me... (So of course I'm looking for another hosting provider...) I have to deal with the occasional 500 errors as well, which, as it turns out, are also largely due to my hosting provider and not my code (but sometimes my code...).

The thing I realized about reliability way too late is this: The utility of a service (or a person) to a person drops off VERY quickly if you dip below 99% reliability (or arguably, 99.9% reliability). Something (or someone) that is 90% reliable is practically useless, because it is bound to fail at the worst possible time. So in all the software I write, I focus on this, and in all my relationships with people (or orgs, or bosses, or my son, or my S.O.), I know that if I commit to them, I must be as reliable as humanly possible because the costs of every slip-up are huge and rise exponentially as they accrue.

An example of a service that is just unreliable enough to be a huge annoyance (which has also caused bad communication failures as well in my experience) is SMS. SMS failures are the worst.

I completely understand that individual browsers may have this or that problem or idiosyncrasy and that may need to be tested but I'd put that in QA, not your official standard app test suite. At the most I'd test whether endpoints actually spit out HTML, CSS etc. but not actually check the rendering step. I do have a JS suite as part of my full test that uses jsdom to check DOM manipulation but it is only because it is not resource-intensive and runs fast.