關(guān)于W3C中對于BFC的定義，有一點小問題想問一下。

W3C中的定義：

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這種例外情況是指的什么情況。