Try it

border-start-start-radius: 80px 80px;

border-start-start-radius: 250px 100px;
direction: rtl;

border-start-start-radius: 50%;
writing-mode: vertical-lr;

border-start-start-radius: 50%;
writing-mode: vertical-rl;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box with a top left rounded corner.
  </div>
</section>

#example-element {
  width: 80%;
  height: 80%;
  display: flex;
  justify-content: center;
  flex-direction: column;
  background-color: #5b6dcd;
  color: white;
  padding: 10px;
}

Syntax

css

/* <length> values */
/* With one value the corner will be a circle */
border-start-start-radius: 10px;
border-start-start-radius: 1em;
/* With two values the corner will be an ellipse */
border-start-start-radius: 1em 2em;
/* Global values */
border-start-start-radius: inherit;
border-start-start-radius: initial;
border-start-start-radius: revert;
border-start-start-radius: revert-layer;
border-start-start-radius: unset;

Values

<length-percentage>: Denotes the size of the circle radius or the semi-major and semi-minor axes of the ellipse. As absolute length it can be expressed in any unit allowed by the CSS <length> data type. Percentages for the horizontal axis refer to the width of the box, percentages for the vertical axis refer to the height of the box. Negative values are invalid.

Formal definition

Initial value	`0`
Applies to	all elements; but User Agents are not required to apply to `table` and `inline-table` elements when `border-collapse` is `collapse`. The behavior on internal table elements is undefined for the moment.. It also applies to `::first-letter`.
Inherited	no
Percentages	refer to the corresponding dimension of the border box
Computed value	two absolute `<length>`s or `<percentage>`s
Animation type	a length, percentage or calc();

Formal syntax

border-start-start-radius = 
  <border-radius>  <border-radius> = 
  <slash-separated-border-radius-syntax>  |
  <legacy-border-radius-syntax>
<slash-separated-border-radius-syntax> = 
  <length-percentage [0,∞]> [ / <length-percentage [0,∞]> ]?
<legacy-border-radius-syntax> = 
  <length-percentage [0,∞]>{1,2}
<length-percentage> = 
  <length>      |
  <percentage>

Examples

Border radius with vertical text

HTML

html

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

CSS

css

div {
  background-color: rebeccapurple;
  width: 120px;
  height: 120px;
  border-start-start-radius: 10px;
}
.exampleText {
  writing-mode: vertical-rl;
  padding: 10px;
  background-color: white;
  border-start-start-radius: 10px;
}

Results

Specifications

Specification
CSS Logical Properties and Values Module Level 1 # border-radius-properties

Browser compatibility

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This page was last modified on Apr 20, 2026 by MDN contributors.