<length>

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

• for absolutely positioned elements, the distance to the left edge of the containing block.
• for relatively positioned elements, the distance that the element is moved to the right 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 right property, while width: auto is treated as a width based on the content; or if right 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 right property; or if right 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 left depends on how the element is positioned (i.e., the value of the position property):

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

When both left and right are defined, and width constraints don't prevent it, the element will stretch to satisfy both. If the element cannot stretch to satisfy both, the position of the element is overspecified. 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.

left = 
  auto                 |
  <length-percentage>  

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

HTML

<div id="wrap">
  <div id="example_1">
    <pre>
      position: absolute;
      left: 20px;
      top: 20px;
    </pre>
    <p>
      The only containing element for this div is the main window, so it
      positions itself in relation to it.
    </p>
  </div>

  <div id="example_2">
    <pre>
      position: relative;
      top: 0;
      right: 0;
    </pre>
    <p>Relative position in relation to its siblings.</p>
  </div>

  <div id="example_3">
    <pre>
      float: right;
      position: relative;
      top: 20px;
      left: 20px;
    </pre>
    <p>Relative to its sibling div above, but removed from flow of content.</p>

    <div id="example_4">
      <pre>
        position: absolute;
        bottom: 10px;
        right: 20px;
      </pre>
      <p>Absolute position inside of a parent with relative position</p>
    </div>

    <div id="example_5">
      <pre>
        position: absolute;
        right: 0;
        left: 0;
        top: 200px;
      </pre>
      <p>Absolute position with both left and right declared</p>
    </div>
  </div>
</div>

CSS

#wrap {
  width: 700px;
  margin: 0 auto;
  background: #5c5c5c;
}

pre {
  white-space: pre;
  white-space: pre-wrap;
  white-space: pre-line;
  word-wrap: break-word;
}

#example_1 {
  width: 200px;
  height: 200px;
  position: absolute;
  left: 20px;
  top: 20px;
  background-color: #d8f5ff;
}

#example_2 {
  width: 200px;
  height: 200px;
  position: relative;
  top: 0;
  right: 0;
  background-color: #c1ffdb;
}
#example_3 {
  width: 600px;
  height: 400px;
  position: relative;
  top: 20px;
  left: 20px;
  background-color: #ffd7c2;
}

#example_4 {
  width: 200px;
  height: 200px;
  position: absolute;
  bottom: 10px;
  right: 20px;
  background-color: #ffc7e4;
}
#example_5 {
  position: absolute;
  right: 0;
  left: 0;
  top: 100px;
  background-color: #d7ffc2;
}

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