Every element in CSS is a rectangular box. Understanding the box model is fundamental to CSS layout.
Box Model Components
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Box Sizing
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Width and Height
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Padding
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Border
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Margin
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Margin Collapse
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Outline
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Display and Box Model
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Overflow
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Box Decoration Break
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Sizing Keywords
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Aspect Ratio
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Visual Formatting Model
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Best Practices
Box Sizing:
✓ Use border-box globally
✓ Understand content-box calculations
✓ Use logical properties for i18n
✓ Reset box-sizing on components if needed
Margins:
✓ Understand margin collapse
✓ Use margin-block for vertical spacing
✓ Use auto margins for centering
✓ Be careful with negative margins
Layout:
✓ Know when to use padding vs margin
✓ Use outline for focus states
✓ Create BFC when needed
✓ Use aspect-ratio for media
Avoid:
✗ Hardcoding heights
✗ Ignoring content-box calculations
✗ Removing focus outlines without alternative
✗ Overusing negative margins
Conclusion
The CSS box model is the foundation of all CSS layouts. Use border-box for predictable sizing, understand margin collapse for vertical spacing, and leverage modern features like aspect-ratio and logical properties. Master these concepts to build robust, maintainable layouts.