From that I assume there's no technical issue with, say, a concrete widget extending more than one mixin itself extending Widget (for instance, Widget's implementation of lifecycle methods wouldn't be called more than once), 
But I'm still wondering about the OOP semantic: by extending Widget, the mixin would clame *being* a Widget, isn't it? Since the class/module name doesn't contain anymore the word "mixin", we'd just count on the doc to clarify for the user that this isn't a concrete widget...Or maybe I'm missing something?