/* ============================================================
   Concept: Opacity
   0 = transparent, 1 = opaque. Affects the whole element.
   ============================================================ */

/* ---------- Base reset & typography ---------- */
* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
body {
  font-family:
    system-ui,
    -apple-system,
    "Segoe UI",
    Roboto,
    sans-serif;
  line-height: 1.6;
  color: #1f2937;
  background: #f8fafc;
  padding: 2rem;
}
.page-header {
  max-width: 760px;
  margin: 0 auto 2rem;
}
.page-header h1 {
  font-size: 1.75rem;
  margin-bottom: 0.5rem;
  color: #0f172a;
}
.page-header .lead {
  color: #475569;
}
.demo {
  max-width: 760px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}
.box {
  padding: 1rem;
  background: #fff;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
}

.op {
  display: inline-block;
  width: 80px;
  height: 80px;
  background: #2563eb;
  color: #fff;
  text-align: center;
  line-height: 80px;
  margin-right: 4px;
}
.o1 {
  opacity: 1;
}
.o7 {
  opacity: 0.7;
}
.o4 {
  opacity: 0.4;
}
.o1s {
  opacity: 0.15;
}