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Markup agnosticism reflects the same basic stance that leads to IoC, Declarative Configuration and Model Transparency: effective organisation of dependencies. In practice, users of a framework want to be able to use their own markup and customize the look and feel of existing components. Virtually every other client-side framework ships their widgets as "black boxes:" markup is baked into code and is invisible to the user. This makes it very difficult to customize the widget without cracking open the code and forking it.
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