Maybe the whole thing here is that we are moving to a component based way to develop UIs with incapsulated and reusable elements but this somehow collides with

Maybe the whole thing here is that we are moving to a component based way to develop UIs with incapsulated and reusable elements but this somehow collides with the foundation of the www i.e. separation of concerns. React, a tool with a very sharp focus (help Facebook to scale), choose to get rid of the whole concern; W3C is trying to solve this with Web Components (Custom Elements are part of them) since 2011 (!) and specs are still in progress and changing from time to time because getting things right is difficult when those concern are at stake.