Yes, it was modern, it maybe still is. Doesn't really care about resolution or window size, with crazy detailed system-wide configurability; set it up once, and all apps that use it look exactly like you want them to. I don't know much about GUI toolkits, other than that most apps don't resize much other than lists and seem very limited that way compared to even basic MUI applications, but I think that MUI kind of puts the modern web and "user agents" to shame, especially if you consider how old it is.