HTML & CSS · The Box Model · Lesson 8 of 25
Width, Height, and Box-Sizing
Concept
Width, Height, and Box-Sizing
box-sizing: border-box makes width and height include padding and border, which avoids a lot of layout surprises.
Without border-box (the default, content-box), padding and border are added on top of the width you set, so a 200px box with 20px padding actually takes up 240px — which trips up almost every beginner at least once.
- Explain the core idea behind Width, Height, and Box-Sizing.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Width, Height, and Box-Sizing. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Compare how a 200px box looks with and without box-sizing: border-box.
Ask about this lesson or your next move. The guide explains the path without taking the work away from you.
Ready for the next lesson?
Mark this lesson complete when you have finished the exercise.
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