Some content in an element may fall outside the element's rendering box for a number of reasons (negative margins, absolute positioning, content exceeding the width/height set for an element, etc.) In cases where this occurs, the 'overflow-y' property describes what to do with the content that exceeds the element's height.
Content is not clipped and may be rendered outside of the element's box.
Content is clipped and content outside of the element's box is not visible. The size of the clipping region is defined by the 'clip' property.
Content is clipped as necessary, but a vertical scrollbar is made available where necessary to view the additional, non-visible content. If the Visual media in use is static (such as Print) the content should be treated as if the value was 'visible'.
This value is browser and media dependent, but should allow for a vertical scrollbar if possible in case of overflow.