Thank you for saying "claims to". Copy pasting a response I made elsewhere:

This is making a mockery of accessibility.

It's the equivalent of having a company that actively discriminates against everyone not passing a internally developed test to check whether you are a "neurotypical" and then respond to criticism by pointing at the wheelchair ramp you installed on one of your entrances.

If I can't copy paste words from your websites to put them into a translator, if I can only use your website when I use one of the two corporate-affiliate-selected screen readers (not even platform-agnostic ones), when I can't look at your website from anything that does not have 4+ cores and a 4G or better connection...saying accessiblity is a "first class citizen" or even citizen at all is laughable at best.

I said "claims to" because I don't know how I can verify if it is. The other person responding to me correctly mention how they suggest tools to scan for accessibility issues on iOS and Android, but they don't say how to do that on the web... have you managed to somehow figure out what is not currently working, exactly?

Regarding performance: Chrome uses Skia to render the DOM, why do you think rendering Flutter, also based on Skia, should be so much worse?

The thing is, when you're shopping for GUI toolkits, "try it and find out" isn't a great decisionmaking rubric. Partially because there are a zillion of them and you simply can't try them all, and partially because learning one is expensive and time-consuming, so, unless you genuinely enjoy that kind of thing, you kind of want to pick one in one shot.

Meaning that, in a slate of options where one of them can definitely do something you need, and another one where the official story seems to be ¯\_(ツ)_/¯ and the examples you've seen seem to indicate that it's at least something you need to go out of your way to accomplish, there really isn't any injustice in deciding to expend no further mental energy on the latter.

That said, if you want spend your own time digging in and finding out, be my guest.

Flutter is not competing with general GUI toolkits, it's competing with fully crossplatform toolkits, of which there's only a couple of real contenders for real-world apps: React Native and maybe Sciter.

Nothing else is production-ready and can run on the web, iOS, Android and desktop OSs (Linux, Windows, MacOS).

It's not competing with pure web frameworks. For this reason, yes, I think that if you're going to criticize it, you absolutely need to know what you're talking about... if Flutter claims it can handle accessibility, it's on you to prove it can't if you make this argument so eloquently on the Internet without actually knowing it.

BTW if you know of any other good UI Toolkits that really can run anywhere like Flutter, using the same code base, please let me know.

No, I just looked at the documentation and saw that it isn't supported on all the platforms I'd be targeting.

For example, that page you link is specific to iOS and Android targets. Nary a mention of what to do when you're targeting the Web. But it's also, at least as of when I did my comparison, explicitly not supported on at least some desktop targets.