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What is the CSS box model? Why is it called a "box"? Let's first talk about some common property names we often hear in web design: content, padding, border, and margin. The CSS box model includes all these properties.
We can transfer these properties to our everyday understanding of boxes (or containers). Boxes we encounter in daily life also have these properties, which is why it's called the "box model." The content refers to what is inside the box; padding is like the foam or other shock-absorbing materials added to protect valuable items inside the box; the border represents the box itself; and the margin indicates that when boxes are placed together, they cannot be stacked directly on top of each other. Instead, there must be some space left for ventilation and ease of access. In web design, content usually refers to elements such as text, images, etc., but it can also be smaller boxes (nested DIVs). Unlike real-life boxes, where objects generally cannot exceed the size of the box without breaking it, CSS boxes are elastic. Even if the contents inside are larger than the box itself, the box will simply expand but will not break. Padding only has a width attribute, which can be understood as the thickness of the shock-absorbing material inside the box. Borders come in different sizes and colors, which can be compared to the thickness and color of the material used to make the box in real life. Margins represent the distance that should be maintained between this box and other objects.
In real life, imagine we are in a square, arranging boxes of different sizes and colors with specific gaps and orders. When viewed from above, the resulting shapes and structures resemble the layout designs we aim to create for web pages, as shown in the figure below.