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+<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8" />
+<meta name="viewport" content="width=device-width,initial-scale=1" />
+<title>3D Triangle (CSS only)</title>
+<style>
+  :root{
+    --size: 260px;      /* width of triangle */
+    --depth: 48px;      /* extrusion depth */
+    --bg: #071029;
+    --face: #06b6d4;
+    --face-2: #038aa6;
+    --shadow: rgba(2,6,23,0.6);
+  }
+
+  html,body{
+    height:100%;
+    margin:0;
+    font-family: system-ui, Arial;
+    background: linear-gradient(180deg, #03243a 0%, var(--bg) 100%);
+    display:grid;
+    place-items:center;
+    color:#fff;
+  }
+
+  .stage{
+    perspective: 900px;
+  }
+
+  /* container that preserves 3D children */
+  .prism {
+    width: var(--size);
+    height: calc(var(--size) * 0.86); /* approx triangle height */
+    position: relative;
+    transform-style: preserve-3d;
+    transform: rotateX(16deg) rotateY(-16deg);
+    transition: transform .45s cubic-bezier(.2,.9,.2,1);
+    filter: drop-shadow(0 18px 30px var(--shadow));
+  }
+
+  .stage:hover .prism{
+    transform: rotateX(6deg) rotateY(18deg) translateZ(8px);
+  }
+
+  /* front & back triangular faces */
+  .face {
+    position: absolute;
+    inset: 0;
+    width: 100%;
+    height: 100%;
+    -webkit-clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
+    clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
+    backface-visibility: hidden;
+  }
+
+  .face.front {
+    background: linear-gradient(180deg, var(--face) 0%, var(--face-2) 100%);
+    transform: translateZ(calc(var(--depth) / 2));
+    box-shadow: 0 6px 18px rgba(3,12,30,.45) inset;
+  }
+
+  .face.back {
+    background: linear-gradient(180deg, var(--face-2) 0%, var(--face) 100%);
+    transform: rotateY(180deg) translateZ(calc(var(--depth) / 2));
+    opacity: 0.96;
+  }
+
+  /* side panels: three trapezoids connecting front & back */
+  .side {
+    position: absolute;
+    width: 100%;
+    height: 100%;
+    transform-origin: center;
+    backface-visibility: hidden;
+  }
+
+  /* left side panel */
+  .side.left {
+    /* create a quadrilateral covering left edge */
+    -webkit-clip-path: polygon(50% 0%, 0% 100%, 0% 100%, 50% 0%);
+    clip-path: polygon(50% 0%, 0% 100%, 0% 100%, 50% 0%);
+    background: linear-gradient(90deg, rgba(0,0,0,.12), rgba(0,0,0,0));
+    transform: rotateY(90deg) translateX(calc(var(--size) / -2)) translateZ(calc(var(--depth) / -2));
+    opacity: 0.95;
+  }
+
+  /* right side panel */
+  .side.right {
+    -webkit-clip-path: polygon(50% 0%, 100% 100%, 100% 100%, 50% 0%);
+    clip-path: polygon(50% 0%, 100% 100%, 100% 100%, 50% 0%);
+    background: linear-gradient(90deg, rgba(0,0,0,0), rgba(0,0,0,.12));
+    transform: rotateY(-90deg) translateX(calc(var(--size) / 2)) translateZ(calc(var(--depth) / -2));
+    opacity: 0.95;
+  }
+
+  /* bottom side panel (base thickness) */
+  .side.bottom {
+    /* a thin band along base */
+    -webkit-clip-path: polygon(0% 100%, 50% 100%, 100% 100%, 100% 100%);
+    clip-path: polygon(0% 100%, 50% 100%, 100% 100%, 100% 100%);
+    background: linear-gradient(180deg, rgba(0,0,0,.12), rgba(255,255,255,0.02));
+    transform: rotateX(90deg) translateY(calc(var(--size) * 0.43)) translateZ(calc(var(--depth) / -2));
+    height: calc(var(--depth));
+    top: auto;
+    bottom: -1px;
+  }
+
+  /* simple label */
+  .label {
+    margin-top: 18px;
+    text-align: center;
+    font-size: 14px;
+    opacity: .85;
+  }
+
+  /* small responsive tweak */
+  @media (max-width:520px){
+    :root{ --size: 200px; --depth: 36px; }
+  }
+</style>
+</head>
+<body>
+  <div class="stage">
+    <div class="prism" aria-hidden="true">
+      <div class="face front"></div>
+      <div class="face back"></div>
+
+      <!-- side panels: visual depth only -->
+      <div class="side left"></div>
+      <div class="side right"></div>
+      <div class="side bottom"></div>
+    </div>
+
+    <div class="label">Pure HTML & CSS — 3D triangle (extruded)</div>
+  </div>
+</body>
+</html>
