Mastering CSS corner-shape with Scroll-Driven Animations

Experimenting With Scroll-Driven corner-shape Animations

The CSS corner-shape property, supported in Chrome 139+, utilizes mathematical superellipse functions to define corner geometry beyond standard rounding. By mapping these shapes to the scroll() animation timeline, developers can synchronize complex geometric morphing with viewport progression.

Why This Matters

While standard border-radius is limited to circular arcs, corner-shape introduces a mathematical foundation that allows for seamless interpolation between distinct geometries like squircles, notches, and bevels. This shift enables high-performance, scroll-synced UI transitions that remain mathematically precise, bypassing the limitations of traditional SVG or clip-path hacks which often fail to scale or animate smoothly across responsive breakpoints.

Key Insights

• The superellipse() function underpins the corner-shape property, allowing keywords like ‘squircle’ to map to superellipse(2) and ‘bevel’ to superellipse(0).
• Scroll-driven animations (part of Interop 2026) tie animation progress directly to scroll position, removing the need for explicit time-based durations.
• The corner-shape property requires Chrome 139+ for full support and must be paired with border-radius to define the coordinate axes for the shape drawing.
• Animating to ‘infinity’ or ‘-infinity’ can cause harsh visual transitions; using finite integers like superellipse(6) provides smoother curvature at viewport edges.
• The mix-blend-mode: difference property can be used with animated shapes to create high-contrast, inverted UI overlays that respond to background content.

Working Examples

Basic scroll-driven corner-shape animation using a pseudo-element and mix-blend-mode.

@keyframes bend-it-like-beckham {
from {
corner-shape: superellipse(-infinity);
}
to {
corner-shape: superellipse(infinity);
}
}
body::before {
content: "";
position: fixed;
inset: -1rem;
pointer-events: none;
mix-blend-mode: difference;
background: white;
border-bottom-left-radius: 100%;
animation: bend-it-like-beckham;
animation-timeline: scroll();
}

Animating multiple nested bevel shapes to create a dynamic diamond-growth effect on scroll.

@keyframes diamonds-are-forever {
from {
padding: 7rem;
}
to {
padding: 14rem;
}
}
#diamonds, #diamonds div {
corner-shape: bevel;
border-radius: 100%;
animation: diamonds-are-forever;
animation-timeline: scroll();
border: 0.0625rem solid #00000030;
}

Practical Applications

• Viewport Masking: Using corner-shape: notch to reveal borders as the user scrolls. Pitfall: Incorrect z-index management can cause the mask to intercept pointer events or hide interactive content.
• Dynamic Geometry: Implementing superellipse(2) for squircle-based cards that transition to squares at scroll endpoints. Pitfall: Animating from extreme infinity values can create a ‘sucking’ visual artifact at the corners.

References:

• https://css-tricks.com/experimenting-with-scroll-driven-corner-shape-animations/