2.1.43 [CSS-Level2-2009] Section 9.4.1, Block formatting contexts
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.
Quirks Mode and IE7 Mode (All Versions)
The following variations apply:
When overflow is set to something other than
visible, table-cell elements do not establish new block formatting contexts.
When overflow is set to
visible,table-cell elements establish a new block formatting context.