CSS corner-block-start-shape Property

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Description

The corner-block-start-shape property controls the geometric profile of an element’s block-start corner in logical coordinates. Rather than leaving the corner as a simple right angle or radius, this property lets authors define a custom corner outline that becomes part of the element’s visible and interactive boundary—affecting painting, hit-testing, and the way nearby content is allowed to intrude or wrap around that corner.

Visually, the corner shape contributes to the element’s outer contour and can change how backgrounds and borders appear at the corner region. It is often used alongside shape-oriented layout features to produce coordinated effects; for example, it can complement the behavior of shape-outside so the element’s corner and its float/wrapping shape align in a coherent way, producing smooth wrapping or artistic cutouts around the element.

Because the property targets a logical corner (the block-start side), its orientation follows the document’s writing direction and block progression, so it interacts with the page’s writing-mode. It can also be combined with clipping operations via clip-path and will influence how text and inline content are prevented from occupying the corner area—behavior that relates to how floated elements behave near that corner described in float.

Practical considerations include accessibility and hit-target expectations: non-rectangular corners change where users can click or how focus outlines are drawn, so designers should test interactive states and keyboard focus. Because complex corner profiles can add compositing or layout cost, use detailed corner shaping selectively and provide sensible fallbacks or simpler visuals for environments that don’t support advanced corner features.

Definition

Initial value: see individual properties
Applies to: see individual properties
Inherited: no
Computed value: see individual properties
Animatable: see individual properties
JavaScript syntax: object.style.cornerBlockStartShape

Syntax

corner-block-start-shape: <corner-shape>{1,2}

Values

The property accepts one or two <corner-shape-value> values. These define the style of the corner shape:

Keyword Values

round - The default value. Creates a standard rounded corner (equivalent to traditional border-radius).
scoop - Creates an inward-facing curve (concave), making the corner look "bitten" out.
bevel - Creates a flat, diagonal cut across the corner (chamfered look).
notch - Creates a rectangular step-like cutout at the corner.
square - Straight angle corner (no rounding)
squircle - Intermediate shape between round and square

Functional Value

superellipse(<number>) - determines corner curvature using a superellipse curve.
- Larger positive numbers → closer to a square
- Values around 1 → default/round
- Negative values → inverted/concave shapes
- infinity and -infinity are allowed as extremes

Global CSS Values

inherit
initial
revert
revert-layer
unset

Example

HTML

<div class="page">

    <div class="corner-fallback">A</div>

    <h1>Chapter 1  -  Getting Started</h1>

    <p>This example shows a decorative corner. On paged media, the @page rule defines a corner shape using corner-block-start-shape.</p>

</div>

CSS

/* Page-level corner shape (for paged media) */
@page {
    size: A4;
    margin: 2cm;

    /* shape for the block-start corner (left side in ltr) */
    corner-block-start-shape: polygon(0% 0%, 100% 0%, 0% 100%);
    corner-block-start-width: 30mm;
    corner-block-start-inset: 0mm;
}

body {
    font-family: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    line-height: 1.4;
    padding: 2rem;
    background: #f7f7fb;
}

.page {
    max-width: 760px;
    margin: 0 auto;
    background: white;
    padding: 2rem 2rem 2rem 4rem; /* leave room for the decorative corner */
    position: relative;
    box-shadow: 0 6px 20px rgba(0, 0, 0, 0.08);
    border-radius: 8px;
}

/* Fallback decorative corner for screen (uses clip-path) */
.corner-fallback {
    position: absolute;
    top: 0;
    left: 0;
    width: 60px;
    height: 60px;
    background: linear-gradient(135deg, #ff6b6b, #ffb86b);
    clip-path: polygon(0 0, 100% 0, 0 100%);
    display: flex;
    align-items: flex-start;
    justify-content: flex-start;
    color: white;
    padding: 8px;
    font-weight: 700;
    border-top-left-radius: 8px;
}

.page h1 {
    margin-top: 0.25rem;
    margin-left: 0.25rem;
}

.page p {
    margin-left: 0.25rem;
    color: #333;
}

Result

Editor

Browser Support

The following information will show you the current browser support for the CSS corner-block-start-shape property. Hover over a browser icon to see the version that first introduced support for this CSS property.