关于W3C中对于BFC的定义，有一点小问题想问一下。

Question

W3C中的定义：

Floats, absolutely positioned elements, block containers (such as inline-blocks, table-cells, and table-captions) that are not block boxes, and block boxes with 'overflow' other than 'visible' (except when that value has been propagated to the viewport) establish new block formatting contexts for their contents.

except when that value has been propagated to the viewport这种例外情况是指的什么情况。