How does CSS create a circle with border-radius and a zero-width/height element?

How does this CSS produce a circle?

The given CSS code produces a circle due to the combination of border-radius and the border properties.

Understanding the CSS properties:

border-radius: This property defines the curvature of the corners of an element's border. In this case, it sets the radius of all four corners to 180px, creating a circular shape.

Border: The border property sets the width, color, and style of an element's border. In this case, it sets the border width to 180px and the color to red.

How it works:

1. The width and height of the element are set to 0px, making it invisible.


2. The 180px border is placed around the element's edges.


3. The border-radius property rounds the corners of the 180px border, creating a circle with a radius of 180px, despite the element's zero width and height.

Visual explanation:

Imagine taking a rectangular box with a 180px width and height (like the green box in the example) and rounding all its corners. As you increase the radius of the rounded corners, the rectangle starts to shrink in size. Eventually, with a radius of 180px, the rectangle completely disappears, leaving only the rounded corners visible as a circle.

Further insights:

• The width and height of an element do not affect the visibility of its border.


• Border-radius applies to the edge of the border, not the content of the element.


• Applying a border-radius value to only two or three corners on a rectangular element creates shapes like ellipses.

Conclusion:

Combining 0px width/height and a 180px border-radius on all corners results in the creation of a circle. Understanding how these CSS properties interact helps in designing visually appealing elements and layouts.

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