The plain-English version
The way an element occupies space through its content, padding, border, and margin.
Why it matters
It explains why a field overflows even when its declared width looks correct.
Let’s make it click
A card with padding and a border may occupy more space than its content width unless its sizing rules account for them.
Vibe Bro’s heads-up
Changing random margins can hide a sizing problem without resolving its cause.
The docs behind it
Documentation links checked .
Editorial review · October 28, 2026
Supported: The standard/content-box versus border-box explanation, spacing example, and overflow caution agree with MDN.