Shadow DOM v0 was kind of a mess (the fragmentation you mention), but v1 kinda washed all that bad taste away and is a pretty sane API. I don't see libraries bu

Shadow DOM v0 was kind of a mess (the fragmentation you mention), but v1 kinda washed all that bad taste away and is a pretty sane API. I don't see libraries building on a feature as "fragmentation," however. Slots are a powerful feature that has to be used carefully. But it's a great API foundation for a lib (which is a tendency for web platform drafts, ex. WebGL and Web Audio.) Customization is just CSS, I'm not sure about the comparison to React. For Shadow DOM styling, we're almost at the point where the last big problem (selective and scalable piercing styles) are getting solved. In any case, the world is better with Web Components than without. Combine them with ES Modules and you have an great ideal for the web platform to strive towards.