CSS Alpha Masking

Meng ToCommunityCreated by Meng To

Skill
Tool integration
Software development
MIT
No issues found

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

Download the skill as a ZIP:
https://mp-staging.autogineer.com/agent-skills/mengto/css-alpha-masking/download

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

Confirm direction (horizontal or vertical) and fade stop percentages.
Provide the inline CSS snippet and any needed class usage.
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

ATTRIBUTION.mdAdded by AutoGineer856 B
LICENSELicence file added by AutoGineer1.0 KiB
SKILL.md1.7 KiB
demo/PROMPT.md1.9 KiB
demo/index.html16.2 KiB
demo/preview.jpg47.7 KiB

demo/index.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="icon" href="data:,">
  <title>CSS Alpha Masking · Skill Demo</title>
  <style>:root{--bg:#080a0f;--panel:rgba(18,21,29,.72);--panelStrong:#121620;--ink:#f2f5f7;--muted:#929aa7;--line:rgba(255,255,255,.14);--accent:hsl(165 92% 68%);font-family:Inter,ui-sans-serif,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;color:var(--ink);background:var(--bg)}
*{box-sizing:border-box}html{background:var(--bg);scroll-behavior:smooth}body{min-width:320px;margin:0;background:radial-gradient(circle at var(--mx,76%) var(--my,16%),color-mix(in srgb,var(--accent) 16%,transparent),transparent 34%),var(--bg);color:var(--ink)}button,a{font:inherit;color:inherit}button{border:1px solid var(--line);border-radius:999px;padding:11px 15px;background:var(--panel);cursor:pointer}button:hover,button:focus-visible{border-color:var(--accent);outline:2px solid color-mix(in srgb,var(--accent) 42%,transparent);outline-offset:3px}.shell{width:min(1280px,calc(100% - 40px));min-height:100svh;margin:auto;padding:22px 0 40px}.topbar{display:flex;align-items:center;justify-content:space-between;padding:10px 0 30px;border-bottom:1px solid var(--line);font-size:11px;font-weight:700;letter-spacing:.13em;text-transform:uppercase}.brand{display:flex;gap:9px;align-items:center}.brand i{width:9px;height:9px;border-radius:50%;background:var(--accent);box-shadow:0 0 24px var(--accent)}.topbar a{text-decoration:none}.intro{display:grid;grid-template-columns:minmax(0,1.25fr) minmax(260px,.75fr);gap:44px;align-items:end;padding:clamp(54px,9vw,116px) 0 42px}.intro small{display:block;margin-bottom:22px;color:var(--muted);font-size:10px;font-weight:700;letter-spacing:.16em;text-transform:uppercase}.intro h1{max-width:11ch;margin:0;font-size:clamp(48px,8vw,112px);font-weight:580;letter-spacing:-.075em;line-height:.86}.intro p{max-width:48ch;margin:0;color:var(--muted);font-size:clamp(14px,1.2vw,18px);line-height:1.6}.showcase{position:relative;min-height:470px;overflow:hidden;border:1px solid var(--line);border-radius:18px;background:var(--panel);isolation:isolate}.showcaseHeader{position:absolute;z-index:5;top:18px;right:18px;left:18px;display:flex;justify-content:space-between;color:var(--muted);font-size:9px;font-weight:700;letter-spacing:.14em;text-transform:uppercase;pointer-events:none}.layoutStudy{display:grid;grid-template-columns:1.15fr .85fr;min-height:470px}.layoutHero{display:flex;flex-direction:column;justify-content:flex-end;padding:clamp(28px,5vw,70px);border-right:1px solid var(--line)}.layoutHero>span,.kicker{color:var(--muted);font-size:10px;font-weight:700;letter-spacing:.14em;text-transform:uppercase}.layoutHero h2,.landing h2{max-width:9ch;margin:18px 0;font-size:clamp(42px,6vw,84px);line-height:.9;letter-spacing:-.065em}.layoutHero p,.landing p{max-width:38ch;color:var(--muted);line-height:1.55}.layoutCards{display:grid;grid-template-rows:repeat(3,1fr)}.layoutCards article{display:flex;align-items:end;justify-content:space-between;padding:26px;border-bottom:1px solid var(--line)}.layoutCards article:last-child{border:0}.layoutCards b{color:var(--accent);font-size:34px}.layoutCards span{text-transform:uppercase;font-size:10px;letter-spacing:.15em}.pricing{display:grid;grid-template-columns:repeat(3,1fr);gap:1px;min-height:470px;background:var(--line)}.pricing article{display:flex;flex-direction:column;padding:80px 28px 28px;background:var(--panelStrong)}.pricing article.featured{background:linear-gradient(160deg,color-mix(in srgb,var(--accent) 20%,var(--panelStrong)),var(--panelStrong))}.pricing p{font-size:11px;letter-spacing:.13em;text-transform:uppercase}.pricing strong{margin:24px 0 8px;font-size:56px;letter-spacing:-.06em}.pricing span{color:var(--muted)}.pricing button{margin-top:auto}.landing{display:flex;min-height:470px;flex-direction:column;justify-content:center;padding:clamp(40px,8vw,100px);background:radial-gradient(circle at 75% 36%,color-mix(in srgb,var(--accent) 45%,transparent),transparent 24%)}.landing .actions{display:flex;gap:10px;margin-top:24px}.landing .actions button:first-child{color:var(--bg);background:var(--ink)}.landing .proof{display:flex;gap:28px;margin-top:54px;color:var(--muted);font-size:11px}.marquee{display:flex;min-height:470px;align-items:center;overflow:hidden;-webkit-mask-image:linear-gradient(90deg,transparent,#000 15%,#000 85%,transparent);mask-image:linear-gradient(90deg,transparent,#000 15%,#000 85%,transparent)}.marqueeTrack{display:flex;width:max-content;animation:marquee 18s linear infinite}.marqueeTrack span{min-width:220px;padding:34px;border:1px solid var(--line);font-size:22px;font-weight:700;letter-spacing:-.04em;text-align:center}.iconGrid,.contactSheet{display:grid;grid-template-columns:repeat(3,1fr);gap:1px;min-height:470px;background:var(--line)}.iconGrid article,.contactSheet article{position:relative;padding:70px 28px 28px;background:var(--panelStrong)}.number{position:absolute;top:22px;right:22px;color:var(--accent);font-size:11px}.glyph{display:grid;width:110px;height:110px;place-items:center;margin:30px auto;border:1px solid var(--line);border-radius:34px;background:linear-gradient(145deg,color-mix(in srgb,var(--accent) 20%,transparent),transparent);font-size:48px}.iconGrid h3{text-align:center}.contactSheet .asset{height:280px;margin-bottom:22px;border-radius:2px;background-size:cover}.assetOne{background:radial-gradient(circle at 70% 30%,#d9f7ff,transparent 20%),linear-gradient(145deg,#203962,#8db3d6 48%,#d7d5c8)}.assetTwo{background:radial-gradient(circle at 50% 28%,#f1c6a2 0 10%,transparent 11%),linear-gradient(120deg,#1b2738,#754c5a 52%,#e1a77f)}.assetThree{background:repeating-radial-gradient(circle at 30% 65%,#c8b8ff 0 9px,#4c4274 10px 18px,#151629 19px 29px)}.contactSheet b,.contactSheet span{display:block}.contactSheet span{margin-top:8px;color:var(--muted);font-size:11px}.surfaceStudy{display:grid;grid-template-columns:1.4fr .6fr;grid-template-rows:1fr auto;gap:18px;min-height:470px;padding:70px 24px 24px}.surfaceStudy article{border:1px solid var(--line);border-radius:20px;background:linear-gradient(145deg,color-mix(in srgb,var(--accent) 12%,var(--panelStrong)),var(--panelStrong))}.surfaceHero{display:flex;grid-row:1/3;flex-direction:column;justify-content:flex-end;padding:42px}.surfaceHero strong{max-width:8ch;margin-top:20px;font-size:clamp(40px,5vw,72px);line-height:.92;letter-spacing:-.06em}.surfaceHero span,.surfaceCard span{color:var(--muted);font-size:10px;letter-spacing:.14em;text-transform:uppercase}.surfaceCard{display:flex;flex-direction:column;padding:26px}.surfaceCard b{margin:auto 0 4px;font-size:56px}.surfaceCard small{color:var(--muted)}.surfaceControl{align-self:end}.motionStage{display:flex;min-height:470px;flex-direction:column;justify-content:center;padding:70px clamp(28px,7vw,90px)}.maskLine{overflow:hidden;margin:0;font-size:clamp(44px,7vw,98px);font-weight:600;letter-spacing:-.075em;line-height:.84}.maskLine span{display:block;transform:translateY(110%);animation:wordIn .8s cubic-bezier(.16,1,.3,1) forwards}.maskLine:nth-child(2) span{animation-delay:.12s}.maskLine:nth-child(3) span{animation-delay:.24s}.accentLine{color:var(--accent)}.timeline{display:flex;gap:8px;margin:42px 0 20px}.timeline i{width:58px;height:3px;background:var(--line);transform-origin:left;animation:grow .8s .42s both}.timeline i:nth-child(2){animation-delay:.54s}.timeline i:nth-child(3){animation-delay:.66s}.timeline i:nth-child(4){animation-delay:.78s}.replay{width:max-content}.visualization{position:relative;min-height:470px}.visualization canvas{position:absolute;inset:0;width:100%;height:100%}.vizMeta{position:absolute;right:28px;bottom:24px;left:28px;display:grid;grid-template-columns:1fr auto 1fr;align-items:end;color:var(--muted);font-size:10px;letter-spacing:.14em;text-transform:uppercase}.vizMeta strong{color:var(--ink);font-size:48px;letter-spacing:-.06em}.vizMeta span:last-child{text-align:right}.simulation{position:relative;min-height:470px}.goo{position:absolute;inset:0;width:100%;height:100%}.goo circle{fill:var(--accent)}.orbA{animation:orbA 4.4s ease-in-out infinite alternate}.orbB{animation:orbB 5.1s ease-in-out infinite alternate}.reset{position:absolute;right:24px;bottom:24px}.demoNote{display:flex;justify-content:space-between;gap:24px;padding:20px 2px;color:var(--muted);font-size:11px;line-height:1.5}.demoNote a{color:var(--ink)}
.pricingWrap{min-height:470px;padding-top:52px}.billingBar{display:flex;align-items:center;justify-content:space-between;min-height:58px;padding:8px 20px;border-top:1px solid var(--line);color:var(--muted);font-size:10px;letter-spacing:.12em;text-transform:uppercase}.billingBar .billingToggle{padding:8px 12px;background:var(--panelStrong);font-size:10px}.pricingWrap .pricing{min-height:360px}.pricingWrap .pricing article{padding:42px 28px 28px}.vizMeta button{justify-self:end;padding:8px 11px;font-size:9px;letter-spacing:.1em;text-transform:uppercase}
.showcase{-webkit-mask-image:linear-gradient(90deg,transparent,#000 14%,#000 86%,transparent);mask-image:linear-gradient(90deg,transparent,#000 14%,#000 86%,transparent)}
@keyframes marquee{to{transform:translateX(-50%)}}@keyframes wordIn{to{transform:translateY(0)}}@keyframes grow{from{transform:scaleX(0)}to{transform:scaleX(1)}}@keyframes orbA{to{transform:translate(-120px,70px)}}@keyframes orbB{to{transform:translate(80px,120px)}}@media(max-width:760px){.intro{grid-template-columns:1fr;padding-top:60px}.intro h1{font-size:54px}.layoutStudy,.pricing,.iconGrid,.contactSheet,.surfaceStudy{grid-template-columns:1fr}.layoutHero{border-right:0;border-bottom:1px solid var(--line)}.pricing article{min-height:320px}.surfaceHero{grid-row:auto}.showcase{min-height:540px}.topbar span:last-child{display:none}.demoNote{flex-direction:column}.landing .proof{flex-direction:column;gap:8px}.vizMeta{grid-template-columns:1fr auto}.vizMeta span:last-child{display:none}}@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}.marqueeTrack,.maskLine span,.timeline i,.orbA,.orbB{animation:none;transform:none}}</style>
</head>
<body data-kind="surface" data-skill="css-alpha-masking">
  <main class="shell">
    <nav class="topbar" aria-label="Demo navigation"><a class="brand" href="#"><i></i>CSS Alpha Masking</a><span>Standalone skill reference · 01</span></nav>
    <header class="intro">
      <div><small>web design / reference demo</small><h1>CSS Alpha Masking</h1></div>
      <p>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.</p>
    </header>
    <section class="showcase" aria-label="CSS Alpha Masking demonstration">
      <div class="showcaseHeader"><span>Live example</span><span>surface</span></div>
<div class="surfaceStudy">
  <article class="surfaceHero"><span>Primary surface</span><strong>Depth without noise.</strong></article>
  <article class="surfaceCard"><span>Compact card</span><b>24</b><small>Active studies</small></article>
  <button class="surfaceControl">Inspect detail</button>
</div>
    </section>
    <footer class="demoNote"><span>Open <strong>PROMPT.md</strong> for the exact recreation and remix prompts.</span><a href="PROMPT.md">Read prompt →</a></footer>
  </main>
  <script>const reduced=matchMedia("(prefers-reduced-motion: reduce)").matches;addEventListener("pointermove",event=>{document.body.style.setProperty("--mx",event.clientX+"px");document.body.style.setProperty("--my",event.clientY+"px")},{passive:true});const billing=document.querySelector(".billingToggle");if(billing)billing.addEventListener("click",()=>{const annual=billing.getAttribute("aria-pressed")!=="true";billing.setAttribute("aria-pressed",String(annual));billing.textContent=annual?"Show monthly pricing":"Show annual pricing";document.querySelectorAll(".pricing strong").forEach(price=>price.textContent=annual?price.dataset.annual:price.dataset.monthly)});const replay=document.querySelector(".replay");if(replay)replay.addEventListener("click",()=>{document.querySelectorAll(".maskLine span,.timeline i").forEach(node=>{node.style.animation="none";node.offsetHeight;node.style.animation=""})});const reset=document.querySelector(".reset");if(reset)reset.addEventListener("click",()=>document.querySelectorAll(".orbA,.orbB").forEach(node=>{node.style.animation="none";node.offsetHeight;node.style.animation=""}));const canvas=document.querySelector("canvas");if(canvas&&!reduced){const context=canvas.getContext("2d");let width=0,height=0,frame=0,time=0;const mode="field";const bodies=Array.from({length:14},(_,i)=>({x:80+(i%7)*92,y:40+Math.floor(i/7)*70,vx:(i%3-1)*.7,vy:0,r:12+(i%4)*5}));const resetBodies=()=>bodies.forEach((body,i)=>{body.x=40+(i%7)*Math.max(32,(width-80)/6);body.y=35+Math.floor(i/7)*58;body.vx=(i%3-1)*.7;body.vy=0});const resetBodiesButton=document.querySelector(".resetBodies");if(resetBodiesButton)resetBodiesButton.addEventListener("click",resetBodies);const resize=()=>{const rect=canvas.getBoundingClientRect();const ratio=Math.min(devicePixelRatio||1,2);width=rect.width;height=rect.height;canvas.width=Math.round(width*ratio);canvas.height=Math.round(height*ratio);context.setTransform(ratio,0,0,ratio,0,0)};const draw=()=>{frame=requestAnimationFrame(draw);time+=.008;context.clearRect(0,0,width,height);const accent=getComputedStyle(document.documentElement).getPropertyValue("--accent").trim();if(mode==="globe"){const radius=Math.min(width,height)*.29;const cx=width*.5,cy=height*.47;context.fillStyle=accent;for(let i=0;i<360;i++){const phi=Math.acos(1-2*(i+.5)/360);const theta=Math.PI*(1+Math.sqrt(5))*i+time;const x=Math.sin(phi)*Math.cos(theta);const y=Math.cos(phi);const z=Math.sin(phi)*Math.sin(theta);const scale=(z+1.7)/2.7;context.globalAlpha=.16+.74*scale;context.beginPath();context.arc(cx+x*radius,cy+y*radius,Math.max(.7,2.2*scale),0,Math.PI*2);context.fill()}context.globalAlpha=.16;context.strokeStyle=accent;context.beginPath();context.ellipse(cx,cy+radius*.18,radius*1.34,radius*.34,time*.4,0,Math.PI*2);context.stroke()}else if(mode==="object"){const size=Math.min(width,height)*.24,cx=width*.5,cy=height*.48;const vertices=[[-1,-1,-1],[1,-1,-1],[1,1,-1],[-1,1,-1],[-1,-1,1],[1,-1,1],[1,1,1],[-1,1,1]];const edges=[[0,1],[1,2],[2,3],[3,0],[4,5],[5,6],[6,7],[7,4],[0,4],[1,5],[2,6],[3,7]];const points=vertices.map(([x,y,z])=>{const a=time*1.1,b=time*.73;const x1=x*Math.cos(a)-z*Math.sin(a),z1=x*Math.sin(a)+z*Math.cos(a),y1=y*Math.cos(b)-z1*Math.sin(b),z2=y*Math.sin(b)+z1*Math.cos(b),p=3.8/(z2+5);return{x:cx+x1*size*p,y:cy+y1*size*p,z:z2,p}});context.strokeStyle=accent;context.lineWidth=1.4;context.shadowColor=accent;context.shadowBlur=12;for(const [a,b] of edges){context.globalAlpha=.25+.45*((points[a].z+points[b].z+4)/8);context.beginPath();context.moveTo(points[a].x,points[a].y);context.lineTo(points[b].x,points[b].y);context.stroke()}context.fillStyle=accent;for(const point of points){context.globalAlpha=.55+.35*(point.z+2)/4;context.beginPath();context.arc(point.x,point.y,3.2*point.p,0,Math.PI*2);context.fill()}context.shadowBlur=0}else if(mode==="matter"){context.fillStyle=accent;for(const body of bodies){body.vy+=.16;body.x+=body.vx;body.y+=body.vy;if(body.x<body.r||body.x>width-body.r){body.vx*=-.94;body.x=Math.max(body.r,Math.min(width-body.r,body.x))}if(body.y>height-body.r-30){body.y=height-body.r-30;body.vy*=-.78}context.globalAlpha=.28+.45*(body.r/32);context.beginPath();context.arc(body.x,body.y,body.r,0,Math.PI*2);context.fill()}context.globalAlpha=.22;context.fillRect(0,height-31,width,1)}else{context.strokeStyle=accent;context.lineWidth=1;for(let i=0;i<18;i++){const y=(i/17)*height;context.globalAlpha=.08+.12*Math.sin(time+i*.4);context.beginPath();for(let x=0;x<=width;x+=12){const wave=Math.sin(x*.012+i*.54+time*3)*18*Math.sin(i/17*Math.PI);context.lineTo(x,y+wave)}context.stroke()}context.globalAlpha=.65;const x=width*(.52+.16*Math.sin(time*.6));const gradient=context.createLinearGradient(x,0,x+2,0);gradient.addColorStop(0,"transparent");gradient.addColorStop(.5,"white");gradient.addColorStop(1,"transparent");context.fillStyle=gradient;context.fillRect(x,0,3,height)}};resize();new ResizeObserver(resize).observe(canvas);const visibility=()=>{if(document.hidden){cancelAnimationFrame(frame);frame=0}else if(!frame)draw()};document.addEventListener("visibilitychange",visibility);draw();}</script>
</body>
</html>

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

2026-11-08

AutoGineer is not affiliated with or endorsed by Meng To.

Download ZIP

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