I'm not sure about the excellent user experience and UIs working well. Im my experience of fixing computers for everyday people with not so fast DSL connection

I'm not sure about the excellent user experience and UIs working well. Im my experience of fixing computers for everyday people with not so fast DSL connections, those javascript based things usually have terrible performance and causes problems by breaking basic web browser functionalities such as history and bookmarks that those people rely on. Then there are the UI issue of too small resolution, or increased font size to help with bad vision and many other accessibility concerns. But I suppose that if you are using a recent computer, on a large flat screen, on high speed internet connection, and do not use privacy related features such as ublock, noscript or policy control you probably enjoy a flawless experience. To me relying only on javascript for your web service or site is akin to answering the customer phone line 0.5% of the time by saying "f* ck you * ssh* le, I sh*t on your ugly face".