I’ve encountered a Flutter web app exactly once, https://app.travellerdeclaration.govt.nz/. It was very painful. (Part of the pain was related to them using a selectable-button style for radio buttons, which isn’t entirely Flutter’s fault, though I bet it contributed to that misguided decision; but when there’s no scrollbar and the Yes/No button you clicked is at the very bottom of the visible page—well, that was the position.) When I filled it out last year, they had an alternative, vastly better accessible version that I think didn’t even require JavaScript, though good luck finding it, since it was accessed by an invisible button that Flutter event handling made unfocusable. Now that button sets a flag in sessionStorage and reloads, and on startup checks for the flag and sets flutterWebRenderer = "html", so that at least the content is in the accessibility tree, even if they still badly reimplement half the stuff the browser provides manually (links, focus, scrolling, they even damage bits of IME).