Let's first break down the CSS:
div {<br> width: 0;<br> height: 0;<br> border: 180px solid red;<br> border-radius: 180px;<br>}<br>
Now, let's think about how these properties work together to create a circle.
The width and height properties apply to the content of an element, not to the border. In this case, since the element has no content, the width and height properties have no effect.
The border-radius property applies to the border of an element. It allows you to round the corners of the border to a specified radius. In this case, the border-radius property is set to 180px, which means the corners of the border will be rounded to a radius of 180 pixels.
When you combine the border and border-radius properties, you can create a circle. The border property creates a rectangular border around the element, and the border-radius property rounds the corners of the border to a specified radius. In this case, the border-radius property is set to 180px, which means the corners of the border will be rounded to a radius of 180 pixels. This creates a circle.
Here is a diagram that shows how the CSS rules in the example work together to create a circle:
In the diagram, the actual content of your element (the little black dot) is really non-existent. If you didn't apply any border-radius you would end up with the green box. The border-radius gives you the blue circle.
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