Giving an element a fixed height and width, then adding significant content to the box, creates a basic overflow example:

The content goes into the box. Once it fills the box, it continues to overflow in a visible way, displaying content outside the box, potentially displaying under subsequent content. The property that controls how overflow behaves is the overflow property which has an initial value of visible. This is why we can see the overflow content.

There are other values that control how overflow content behaves. To hide overflowing content use a value of hidden. This may cause some of your content to not be visible.

Using a value of scroll contains the content in its box and add scrollbars to enable viewing it. Scrollbars will be added even if the content fits in the box.

Using a value of auto will display the content with no scrollbars if the content fits inside the box. If it doesn't fit then scrollbars will be added. Comparing the next example with the example for overflow: scroll you should see overflow scroll has horizontal and vertical scrollbars when it only needs vertical scrolling. The auto example below only adds the scrollbar in the direct we need to scroll.

As we have already learned, using any of these values, other than the default of visible, will create a new Block Formatting Context.

Note: In the Working Draft of Overflow Level 3, there is an additional value overflow: clip. This will act like overflow: hidden however it does not allow for programmatic scrolling, the box becomes non-scrollable. In addition it does not create a Block Formatting Context.

The overflow property is in reality a shorthand for the overflow-x and overflow-y properties. If you specify only one value for overflow, this value is used for both axes. However, you can specify both values in which case the first is used for overflow-x and therefore the horizontal direction, and the second for overflow-y and the vertical direction. In the below example, I have only specified overflow-y: scroll so we do not get the unwanted horizontal scrollbar.

In the guide to Writing Modes and Flow Layout, we looked at the newer properties of block-size and inline-size which make more sense when working with different writing modes than tying our layout to the physical dimensions of the screen. The Level 3 Overflow Module also includes flow relative properties for overflow - overflow-block and overflow-inline. These correspond to overflow-x and overflow-y but the mapping depends on the writing mode of the document.

These properties currently do not have implementations in browsers, so you will need to use the physical properties at the present time and adjust for your writing mode.

In the Level 3 Overflow specification we have some properties which can help improve the way content looks in an overflow situation.

The text-overflow property deals with text overflowing in the inline direction. It takes one of two values clip, in which case content is clipped when it overflows, this is the initial value and therefore the default behavior. We also have ellipsis which renders an ellipsis, which may be replaced with a better character for the language or writing mode in use.

There is also a proposal for a block-overflow property, although at the time of writing the name is still up for discussion. This proposal would enable the adding of an ellipsis when text overflows in the block dimension.

This is useful in the situation where you have a listing of articles, for example, and you display the listings in fixed height boxes which only take a limited amount of text. It may not be obvious to the reader that there is more content to click through to when clicking the box or the title. An ellipsis indicates clearly the fact there is more content. The specification would allow a string of content or a regular ellipsis to be inserted.

Whether you are in continuous media on the web or in a Paged Media format such as print or EPUB, understanding how content overflows is useful when dealing with any layout method. By understanding how overflow works in normal flow, you should find it easier to understand the implications of overflow content in layout methods such as grid and flexbox.