HTML.CSS.BoxModel

Here is the box model

this will squeze box to definite size, for example 300 px width

box {
  width: 300px;
  box-sizing: border-box;
}

And this will set content width to 300 px

box {
  width: 300px;
  box-sizing: content-box;
}

There is a star selector that applies css rule to every element

* {
   box-sizing: content-box;
}

Horizontal margins are summed, Vertical margins are combined (from 30 px. and 20 px. will be chosen 30 px., the biggest)