Styla saysbox-sizing: border-box makes width include padding and border — saner box math.
Set box-sizing: border-box and width now includes padding and border, so a 200px box stays 200px no matter what you add inside. Nearly every project applies it globally: *{ box-sizing: border-box }. It's the single most quality-of-life CSS reset.
Power-ups you unlock
border-box → width includes padding + border
Set globally: *{box-sizing:border-box}
A 200px box stays 200px
The default (content-box) excludes them
The Specificity Sphinx attacks — common mistakes
Not applying it and fighting box math all day
Applying it to some elements but not others
Forgetting it affects height too
Boss battleAdd *{box-sizing:border-box} and re-check the padded, bordered box is exactly 200px.