`inset-block-start` CSS property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It's been available across browsers since September 2021.

The inset-block-start CSS property defines the logical block start offset of an element, which maps to a physical inset depending on the element's writing mode, directionality, and text orientation. It corresponds to the top, right, bottom, or left property depending on the values defined for writing-mode, direction, and text-orientation.

This inset property has no effect on non-positioned elements.

Try it

writing-mode: horizontal-tb;

writing-mode: vertical-rl;

writing-mode: horizontal-tb;
direction: rtl;

writing-mode: vertical-lr;

<section id="default-example">
  <div class="example-container" id="example-element">
    <div id="abspos">
      I am absolutely positioned with inset-block-start: 50px
    </div>
    <p>
      As much mud in the streets as if the waters had but newly retired from the
      face of the earth, and it would not be wonderful to meet a Megalosaurus,
      forty feet long or so, waddling like an elephantine lizard up Holborn
      Hill.
    </p>
  </div>
</section>

#example-element {
  border: 0.75em solid;
  padding: 0.75em;
  position: relative;
  width: 100%;
  min-height: 200px;
  unicode-bidi: bidi-override;
}

#abspos {
  background-color: yellow;
  color: black;
  border: 3px solid red;
  position: absolute;
  inset-block-start: 50px;
  inline-size: 140px;
}

Syntax

/* <length> values */
inset-block-start: 3px;
inset-block-start: 2.4em;
inset-block-start: anchor(end);
inset-block-start: calc(anchor-size(--my-anchor height, 70px) * 2);

/* <percentage>s of the width or height of the containing block */
inset-block-start: 10%;

/* Keyword value */
inset-block-start: auto;

/* Global values */
inset-block-start: inherit;
inset-block-start: initial;
inset-block-start: revert;
inset-block-start: revert-layer;
inset-block-start: unset;

Values

The inset-block-start property takes the same values as the top property.

Formal definition

Initial value	auto
Applies to	positioned elements
Inherited	no
Percentages	logical-height of containing block
Computed value	same as box offsets: `top`, `right`, `bottom`, `left` properties except that directions are logical
Animation type	$1$, <a href="/en-US/docs/Web/CSS/Reference/Values/percentage#interpolation" title="Values of the <percentage> CSS data type are interpolated as real, floating-point numbers.">percentage</a> or calc();

Formal syntax

inset-block-start = 
  <'top'>

Examples

Setting block start offset

HTML

<div>
  <p class="exampleText">Example text</p>
</div>

CSS

div {
  background-color: yellow;
  width: 120px;
  height: 120px;
}

.exampleText {
  writing-mode: vertical-lr;
  position: relative;
  inset-block-start: 20px;
  background-color: #c8c800;
}

Result

Specifications

Specification
CSS Logical Properties and Values Module Level 1 # propdef-inset-block-start
CSS Positioned Layout Module Level 3 # propdef-inset-block-start

Browser compatibility

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`inset-block-start`	87	87	63 41–63	73	14.1	87	63 41–63	62	14.5	14.0	87	14.5
`anchor`	125	125	147	111	26	125	147	83	26	27.0	125	26
`anchor-size`	132	132	147	117	26	132	147	87	26	29.0	132	26
`auto`	87	87	41	73	14.1	87	41	62	14.5	14.0	87	14.5