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
topproperty - 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
Data on support for the mdn-css__properties__inset-block-start feature across the major browsers
Demo
Related properties
Further reading
- CSS Logical Properties and Values Level 1 specification (Editor’s Draft)
- MDN Documentation
- Understanding Logical Properties And Values (Smashing Magazine)
- CSS Logical Properties (Adrian Roselli)
- Bidirectional horizontal rules in CSS (Hussein Al Hammad)
