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.
<blockquote style="width: 50px; height: 50px;
overflow-y: scroll">some text</blockquote>
visible: Content is not clipped and may be rendered outside of the element’s box.
hidden: 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.
scroll: 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’.
auto: This value is browser and media dependent, but should allow for a vertical scrollbar if possible in case of overflow.