Imagine something like these Transformer Tabs as a widget in a fluid column in a responsive design. Depending on the browser window width, perhaps this design is either 4, 2, or 1 column wide. When it breaks from 4 to 2, the column probably temporarily gets wider than it was, even though the screen is narrower. It would be preferable when writing the media query logic for those tabs to consider how much space the widget has available rather than the entire window, which might be totally unrelated, especially when re-using this widget.
Jonathan Neal has some thoughts on how this might work, including the complicated bits you might not have thought about, like how a widgets contents might affect its parent container and cause an infinite loop.