The effect of bottom depends on how the element is positioned (i.e., the value of the position property):

• When position is set to absolute or fixed, the bottom property specifies the distance between the element's outer margin of bottom edge and the inner border of the bottom edge of its containing block.
• When position is set to relative, the bottom property specifies the distance the element's bottom edge is moved above its normal position.
• When position is set to sticky, the bottom property is used to compute the sticky-constraint rectangle.
• When position is set to static, the bottom property has no effect.

When both top and bottom are specified, position is set to absolute or fixed, and height is unspecified (either auto or 100%) both the top and bottom distances are respected. In all other situations, if height is constrained in any way or position is set to relative, the top property takes precedence and the bottom property is ignored.

<length>

A negative, null, or positive <length> that represents:

• for absolutely positioned elements, the distance to the bottom edge of the containing block.
• for relatively positioned elements, the distance that the element is moved above its normal position.

<percentage>

A <percentage> of the containing block's height.

auto

Specifies that:

• for absolutely positioned elements, the position of the element is based on the top property, while height: auto is treated as a height based on the content; or if top is also auto, the element is positioned where it should vertically be positioned if it were a static element.
• for relatively positioned elements, the distance of the element from its normal position is based on the top property; or if top is also auto, the element is not moved vertically at all.

inherit

Specifies that the value is the same as the computed value from its parent element (which might not be its containing block). This computed value is then handled as if it were a <length>, <percentage>, or the auto keyword.

bottom = 
  auto                 |
  <length-percentage>  

<length-percentage> = 
  <length>      |
  <percentage>  

This example demonstrates the difference in behavior of the bottom property, when position is absolute versus fixed.

HTML

<p>
  This<br />is<br />some<br />tall,<br />tall,<br />tall,<br />tall,<br />tall<br />content.
</p>
<div class="fixed"><p>Fixed</p></div>
<div class="absolute"><p>Absolute</p></div>

CSS

p {
  font-size: 30px;
  line-height: 2em;
}

div {
  width: 48%;
  text-align: center;
  background: rgba(55, 55, 55, 0.2);
  border: 1px solid blue;
}

.absolute {
  position: absolute;
  bottom: 0;
  left: 0;
}

.fixed {
  position: fixed;
  bottom: 0;
  right: 0;
}

Result

• inset, the shorthand for all related properties: top, bottom, left, and right
• The mapped logical properties: inset-block-start, inset-block-end, inset-inline-start, and inset-inline-end and the shorthands inset-block and inset-inline
• position