How does border-image-slice work with a gradient background in CSS?

Border-Image in Gradient Backgrounds: Understanding Number 80

In CSS, border-image allows us to use a raster or vector image as a border around an element. When applied to a gradient background, as seen in the provided code, the syntax may leave some questions about how the border-image-slice property works.

According to the CSS specifications, border-image-slice represents an edge offset in pixels for raster images. However, in the case of a gradient background, it is relative to the element's size.

In the example provided, the border-image-slice value is set to 80. This value is unitless, which means it's assumed to be in pixels. So, in this case, 80px is the size of the image that will be used to create the border.

The border-image-width property defines the width of the border. In this example, it is set to 5em, which is roughly 5 times the font size of the element. This means that the border will be 5em wide on all sides.

The key to understanding how these values interact is to realize that the initial image is scaled to fit the size of the border. In this case, the initial image is the gradient background. So, the gradient background will be scaled to be 80px wide and 5em tall.

The border-image-slice property then defines how this scaled image is sliced into nine regions. These regions are then placed around the element as follows:

• The top and left regions are placed as the top/left corners of the border.
• The bottom and right regions are placed as the bottom/right corners of the border.
• The regions on either side of the top/bottom are stretched to fit the width of the border.
• The regions on either side of the left/right are stretched to fit the height of the border.

So, the border-image-slice value of 80px in this example means that the scaled gradient background will be sliced into nine 80px squares. These squares will then be placed around the element as described above to form the border.

By adjusting the border-image-slice and border-image-width values, you can control the appearance of the border. Experimenting with different values can help you achieve the desired effect for your design.

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