<length>

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

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

<percentage>

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

auto

Specifies that:

• for absolutely positioned elements, the position of the element is based on the left property, while width: auto is treated as a width based on the content; or if left is also auto, the element is positioned where it should horizontally 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 left property; or if left is also auto, the element is not moved horizontally 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.

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

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

When both left and right are defined, if not prevented from doing so by other properties, the element will stretch to satisfy both. If the element cannot stretch to satisfy both — for example, if a width is declared — the position of the element is over-constrained. When this is the case, the left value has precedence when the container is left-to-right; the right value has precedence when the container is right-to-left.

right = 
  auto                 |
  <length-percentage>  

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

HTML

<div id="relative">Relatively positioned</div>
<div id="absolute">Absolutely positioned</div>

CSS

#relative {
  width: 100px;
  height: 100px;
  background-color: #ffc7e4;
  position: relative;
  top: 20px;
  left: 20px;
}

#absolute {
  width: 100px;
  height: 100px;
  background-color: #ffd7c2;
  position: absolute;
  bottom: 10px;
  right: 20px;
}

Result

When both left and right are declared, the element will stretch to meet both, unless other constraints prevent it from doing so. If the element will not stretch or shrink to meet both. When the position of the element is overspecified, the precedence is based on the container's direction: The left will take precedence if the container's direction is left-to-right. The right will take precedence if the container's direction is right-to-left.

HTML

<div id="parent">
  Parent
  <div id="noWidth">No width</div>
  <div id="width">width: 100px</div>
</div>

CSS

div {
  outline: 1px solid #cccccc;
}
#parent {
  width: 200px;
  height: 200px;
  background-color: #ffc7e4;
  position: relative;
}
/* declare both a left and a right */
#width,
#noWidth {
  background-color: #c2ffd7;
  position: absolute;
  left: 0;
  right: 0;
}
/* declare a width */
#width {
  width: 100px;
  top: 60px;
}

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