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inset-block-start is a logical CSS property that sets the length that an element is offset in the block direction from its starting edge. It’s sort of like declaring top except its starting point is determined by the element’s direction, text-orientation and writing-mode, just like other logical properties.

The property is part of the CSS Logical Properties and Values Level 1 specification which is currently in Editor’s Draft status. That means the definition and information about it can change between now and official recommendation.

.element {
  inset-block-start: 50px;
  position: relative; /* Apples to positioned elements */
  writing-mode: vertical-rl; /* Determines the block start direction */
}

So, for example, if the writing mode is set to horizontal-lr the inset-block-start property will act just like top and set the element’s offset from it’s starting edge, which is the top. You even need to specify an explicit position on the same element for it to take effect, just like top and other physical offset properties.

But change the element’s writing-mode to something like vertical-rl and the starting edge is rotated in the vertical direction, acting more like the left property.

Syntax

inset-block-start: <'top'>;
  • Initial value: auto
  • Applies to: positioned elements
  • Inherited: no
  • Percentages: as for the corresponding physical property
  • Computed value: same as corresponding top property
  • Animation type: by computed value type

Values

inset-block-start takes a length value and supports global keywords. Its default value is auto.

/* Length values */
inset-block-start: 50px;
inset-block-start: 4em;
inset-block-start: 3.5rem
inset-block-start: 25vh;

/* Percentage values */
inset-block-start: 50%;

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

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

Browser support

IEEdgeFirefoxChromeSafariOpera
NoNo63+NoNoNo
Android ChromeAndroid FirefoxAndroid BrowseriOS SafariOpera Mini
No79+NoNoNo
Source: caniuse

Demo

Related properties

Further reading

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