This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for the proper prefixes to use in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the spec changes.


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

It relates to border-block-end-widthborder-inline-start-width, and border-inline-end-width, which define the other border widths of the element.

Initial valuemedium
Applies toall elements
Percentageslogical-width of containing block
Computed valueabsolute length; 0 if the border style is none or hidden
Canonical orderthe unique non-ambiguous order defined by the formal grammar


/* One-value syntax */
border-block-start-width: width
border-block-start-width: 5px;

/* Two-value syntax */
border-block-start-width: horizontal vertical
border-block-start-width: 0.3em 0 9px;

/* Three-value syntax */
border-block-start-width: top horizontal bottom
border-block-start-width: 0.3em 0 9px thick;

/* Four-value syntax */
border-block-start-width: top right bottom left
border-block-start-width: 0.3em 0 9px thick 1em;

border-block-start-width: inherit


See border-width

Formal syntax



HTML Content

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

CSS Content

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

.exampleText {
  writing-mode: vertical-lr;
  border: 1px solid blue;
  border-block-start-width: 5px;


Specification Status Comment
CSS Logical Properties Level 1
The definition of 'border-block-start-width' in that specification.
Editor's Draft Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support Not supported 41.0 (41.0)[1] Not supported Not supported Not supported
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support Not supported 41.0 (41.0)[1] Not supported Not supported Not supported

[1] Available since Gecko 38, but behind the preference layout.css.vertical-text.enabled, then disabled by default.

See also

Document Tags and Contributors

 Contributors to this page: Sebastianz, fscholz, teoli, jsx, kscarfone
 Last updated by: Sebastianz,