Parent div with 50% width.
Child div with 90% width, 4px black border, and 15% padding
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
Child div with 50% width, 4px black border, and 1em padding
Lord, have mercy.
Child div with 50% width, 4px black border, and 1em padding
Glory to the Father, and to the Son, and to the Holy Spirit, both now and ever, and unto the ages of ages. Amen.
box-sizing: border-box
. Vanilla html's default is content-box
.