All of the components of the box model comprise an element’s size. For example, an image that has the following dimensions is 364 pixels wide and 244 pixels tall.

• 300 pixels wide
• 200 pixels tall
• 10 pixels padding on the left and right
• 10 pixels padding on the top and bottom
• 2 pixels border on the left and right
• 2 pixels border on the top and bottom
• 20 pixels margin on the left and right
• 10 pixels margin on the top and bottom

The total dimensions (364px by 244px) are calculated by adding all of the vertical dimensions together and all of the horizontal dimensions together. Sometimes, these components result in an element that is larger than the parent’s containing area.

How can we ensure that we can view all of an element that is larger than its parent’s containing area?

The overflow property controls what happens to content that spills, or overflows, outside its box. The most commonly used values are:

• hidden—when set to this value, any content that overflows will be hidden from view.
• scroll—when set to this value, a scrollbar will be added to the element’s box so that the rest of the content can be viewed by scrolling.
• visible—when set to this value, the overflow content will be displayed outside of the containing element. Note, this is the default value.

p {
  overflow: scroll; 
}

In the example above, if any of the paragraph content overflows (perhaps a user resizes their browser window), a scrollbar will appear so that users can view the rest of the content.

The overflow property is set on a parent element to instruct a web browser on how to render child elements. For example, if a div’s overflow property is set to scroll, all children of this div will display overflowing content with a scroll bar.

For a more in-depth look at overflow, including additional properties like overflow-x and overflow-y that separate out the horizontal and vertical values, head over to the MDN documentation.