CSS border-block-start-width property


CSS border-block-start-width is a logical property that defines width for the start side border of a block. The selection of start side depends upon the writing-mode.

  • The start side represents top side for the horizontal-tb writing-mode.

  • The start side represents right or left side for the vertical-rl or vertical-lr writing-mode respectively.

Change the direction of flow of text.

horizontal-tb vertical-lr vertical-rl


border-block-start-width : medium | thin | thick | +ve length

Property values


medium value represents medium sized border. thin ≤ medium ≤ thick

border-block-start-width: medium;


thin value defines thin line border.

border-block-start-width: thin;


thick value defines a thick line border.

border-block-start-width: thick;

+ve length

length defines the line width of border. The used units might be px, in, em, pt.

border-block-start-width: 10px;

Applicable to

It applies to all elements

#CSS digger
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Check the behavior of border-block-start-width for different value of writing mode.