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The Box Model

The box model is made up of content, padding, border and margin.

  • Content: The actual text / child elements in an HTML node <div>I'm the content</div>.
  • Padding: The space around the content.
  • Border: Surrounds the padding.
  • Margin: Outside the border. The space between elements.

Box Sizing

By default, browser's apply border and padding on top of an element's height and width. Avoid this behavior with box-sizing: border-box;.

Padding

  • Inner space between content and border.
  • Background color applied to an element will be applied to the padding.
  • Can be set all at once padding: 5px;, per side padding-top: 5px; or per logical property padding-block-start: 5px; (start is the left side in English).

Border

  • Consists of width, style (dotted, solid, etc.), color.
  • Can take a shorthand border: 10px dotted red;.
  • Defaults to the color of the content. Can be set to the color of the content explicitly with border-color: currentColor;.
  • border-radius is really more of a corner-radius because it rounds the corners regardless of a border's existence.
  • outline can be used in a similar way. Outline does not affect layout and is on the outside of the border.
  • Use outline-offset to create a gap between the outline and border.

Margin

  • Adds space between elements.
  • Syntax is the same as padding.
  • Accepts negative values to move an element outside its parent or move siblings closer.
  • Center an element with margin: auto; (only works with horizontal margin).
    • If applied for vertical margin it turns to 0 margin.
    • Only works if an element has an explicit width.
  • Setting negative margin to parent padding will allow the child to break out to the edge of the parent.
  • Margin between elements collapses, so if two elements have 20px margin then the total between them will be 20px not 40px.

Margin Collapse Rules

  • Only applies for vertical margin.
  • Only collapses in Flow Layout.
  • Only adjacent elements collapse. Adding a br tag will stop the collapse.
  • The bigger margin wins.
    • Element A has margin-bottom: 20px; and Element B has margin-top: 10px; the total margin between them is only 20px.
  • Nesting doesn't prevent margin collapse. Margin will transfer to the parent if it can to maintain this behavior.
  • Margins can only collapse if they touch. Adding border, padding or even whitespace from a larger parent height stops the collapse.
  • Margins can collapse in the same direction (parent and child both have top margin). The larger margin will win.
  • More than two margins can collapse.
<--
The parent and child margins for A collapse
into the parent and child margins for B
-->

<div style="margin-bottom: 5px;">
  <p style="margin-bottom: 20px;">A</p>
</div>
<div style="margin-top: 5px;">
  <p style="margin-top: 20px;">B</p>
</div>
  • Negative margins collapse in reverse.
    • Element A has margin-bottom: -20px; and Element B has margin-top: -10px; the total margin between them is only -20px.
  • A mix of positive and negative margins will be added together. A -25px margin and 25px margin will cancel out.
  • Multiple positive and negative margins works the same way. The largest positive and smallest negative margin are found and added together.