30 seconds of CSS

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Box-sizing reset  layout

Resets the box-model so that widths and heights are not affected by their borders or padding.

HTML
<div class="box">border-box</div>
<div class="box content-box">content-box</div>
CSS
html {
  box-sizing: border-box;
}
*,
*::before,
*::after {
  box-sizing: inherit;
}
.box {
  display: inline-block;
  width: 150px;
  height: 150px;
  padding: 10px;
  background: tomato;
  color: white;
  border: 10px solid red;
}
.content-box {
  box-sizing: content-box;
}
Demo
border-box
content-box
Explanation
  1. box-sizing: border-box makes the addition of padding or borders not affect an element's width or height.
  2. box-sizing: inherit makes an element respect its parent's box-sizing rule.
Browser support

100.0%