11.1 Overflow and clipping

Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box, e.g.:

• A line cannot be broken, causing the line box to be wider than the block box.
• A block-level box is too wide for the containing block. This may happen when an element's 'width' property has a value that causes the generated block box to spill over sides of the containing block.
• An element's height exceeds an explicit height assigned to the containing block (i.e., the containing block's height is determined by the 'height' property, not by content height).
• A descendant box is positioned absolutely, partly outside the box. Such boxes are not always clipped by the overflow property on their ancestors; specifically, they are not clipped by the overflow of any ancestor between themselves and their containing block
• A descendant box has negative margins, causing it to be positioned partly outside the box.
• The 'text-indent' property causes an inline box to hang off either the left or right edge of the block box.

Whenever overflow occurs, the 'overflow' property specifies whether a box is clipped to its padding edge, and if so, whether a scrolling mechanism is provided to access any clipped out content.

11.1.1 Overflow: the 'overflow' property