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CSS Alpha Masking

Meng ToCommunityCreated by Meng To

  • Skill
  • Tool integration
  • Software development
  • MIT
  • No issues found
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From the author

Apply CSS alpha masking with linear-gradient for horizontal or vertical edge fades (mask-image and -webkit-mask-image). Use when asked for alpha masks, fade edges, or CSS mask gradients.
— the description by Meng To

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a standardized approach to implementing CSS alpha masking using linear-gradient properties. It ensures cross-browser compatibility by including both standard mask-image and vendor-prefixed -webkit-mask-image properties, allowing developers to create smooth horizontal or vertical fades for images, text, or containers.

What it does

  • Generates CSS snippets for horizontal and vertical edge fading.
  • Ensures Safari compatibility by providing the necessary vendor-prefixed properties.
  • Offers guidance on adjusting fade depth and intensity using percentage stops and alpha values.
  • Provides a structured workflow for gathering requirements like direction and fade width.

When to use it

  • When you need to fade the edges of an image or container into transparency.
  • When implementing UI effects that require smooth transitions at element boundaries.
  • When you need a quick, copy-pasteable CSS solution for alpha masking.
  • When you need to ensure your CSS masks work consistently across different browsers.

Advantages

  • Reduces manual CSS configuration time for common masking patterns.
  • Prevents common cross-browser issues by including required vendor prefixes.
  • Provides clear customization options for fine-tuning fade intensity and direction.
  • Standardizes the implementation of visual edge effects across a project.

Usage examples

“Create a horizontal fade for my image gallery container.”
The agent provides the CSS snippet using linear-gradient(to right, ...) with both standard and webkit properties to ensure the container fades out at both ends.
“How do I make a top-to-bottom fade that is very subtle?”
The agent provides the vertical mask CSS and suggests changing the transparent stop to a semi-transparent color like rgba(0,0,0,0.2) for a softer effect.

Install

  1. Download the skill as a ZIP:

    https://autogineer.com/agent-skills/mengto/css-alpha-masking/download

  2. Unzip it into the cross-client skills folder of your project:

    mkdir -p .agents/skills && unzip css-alpha-masking.zip -d .agents/skills

Documentation by Meng To

CSS Alpha Masking Skill

Workflow

  1. Confirm direction (horizontal or vertical) and fade stop percentages.

  2. Provide the inline CSS snippet and any needed class usage.

  3. Offer small tweaks only (direction, stop positions, colors).

Usage checklist

  • Apply the mask styles directly on the element or in a CSS class.

  • Always include both mask-image and -webkit-mask-image for Safari.

  • Ensure the element has visible content; masks reveal/hide alpha only.

Horizontal (left/right) fade

/* Add this inline CSS to any element */
mask-image: linear-gradient(to right, transparent, black 15%, black 85%, transparent);
-webkit-mask-image: linear-gradient(to right, transparent, black 15%, black 85%, transparent);

Vertical (top/bottom) fade

/* Add this inline CSS to any element */
mask-image: linear-gradient(to bottom, transparent, black 15%, black 85%, transparent);
-webkit-mask-image: linear-gradient(to bottom, transparent, black 15%, black 85%, transparent);

Customization knobs

  • Direction: to right, to left, to bottom, to top.

  • Fade depth: adjust 15% and 85% stops.

  • Strength: change transparent to rgba(0,0,0,0.2) for softer fades.

Common pitfalls

  • Forgetting the -webkit-mask-image fallback in Safari.

  • Expecting masks to work on elements with overflow: hidden but no visible content behind.

Questions to ask when specs are missing

  • Which direction should the fade go?

  • How wide should the fade edges be?

  • Is this for images, text, or a container background?

Files

demo/PROMPT.md

CSS Alpha Masking Demo Prompts

Minimal prompt

Use $css-alpha-masking to create a polished standalone HTML example that clearly demonstrates the skill.

Recreate the demo

Use $css-alpha-masking to build a responsive reference demo for this capability:

Apply CSS alpha masking with linear-gradient for horizontal or vertical edge fades (mask-image and -webkit-mask-image). Use when asked for alpha masks, fade edges, or CSS mask gradients.

Direction

Compare the treatment across a hero surface, compact control, and detail card.

Use an art-directed composition with one clear focal point, restrained supporting copy, and enough surrounding interface to show how the technique behaves in a real product or landing page.

Canonical example

  • Show the treatment on a large Depth without noise surface, one compact metric card, and one control.

  • Use the same edge, shadow, blur, or masking logic consistently across all three scales.

Deliverable

  • Create demo/index.html as a standalone document.

  • Keep CSS and JavaScript inline.

  • Put any required images, fonts, models, textures, or vendor files in demo/assets/.

  • Do not add a framework, package manager, build step, or node_modules.

  • Add controls only when they help inspect or replay the technique.

Acceptance checks

  • Keep the demo responsive from 390px through 1440px.

  • Use semantic HTML, visible focus states, and reduced-motion handling.

  • Keep all HTML, CSS, and JavaScript inside demo/index.html.

  • Use only relative local asset paths.

  • Preserve readable contrast at the softest and strongest treatment levels.

  • Avoid clipping focus rings or interactive content.

Remix prompt

Use $css-alpha-masking and keep the same implementation contract, but change the subject, copy, palette, and content hierarchy. Preserve the core technique, accessibility behavior, responsive rules, and performance constraints demonstrated by the reference.

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Security

No issues found by our static scan· No issues found

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Created by
Meng To
Publisher
Meng To (Community)
Source
MengTo/Skills · agent-skills/web-design/css-alpha-masking · commit 5ea1661
Licence
MIT License (redistribution allowed)
  • LICENSE (added by AutoGineer from LICENSE)
  • ATTRIBUTION.md (written by AutoGineer)
Synced

AutoGineer is not affiliated with or endorsed by Meng To or Meng To.

Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download css-alpha-masking.zip