body {
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
  margin: 0;
  display: flex;
  background: #f9f9f9;
}

.sidebar {
  width: 220px;
  background: #222;
  color: white;
  height: 100vh;
  position: fixed;
  left: 0;
  top: 0;
  padding-top: 1rem;
  overflow-y: scroll;
}

.sidebar h3 {
  text-align: center;
  margin-bottom: 1rem;
}

.sidebar a {
  display: block;
  padding: 0.8rem 1rem;
  color: #ccc;
  text-decoration: none;
  border-left: 4px solid transparent;
  transition: background 0.2s, color 0.2s;
}

.sidebar a:hover {
  background: #333;
  color: white;
}

.sidebar a.active {
  background: #007bff;
  color: white;
  border-left: 4px solid #00bfff;
}

.main {
  margin-left: 220px;
  padding: 2rem;
  text-align: center;
  flex-grow: 1;
}

video {
  width: 80%;
  max-width: 800px;
  border-radius: 12px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.2);
  margin-bottom: 1rem;
}

.nav-links {
  display: flex;
  justify-content: center;
  gap: 1rem;
}

a.nav-btn {
  background: #007bff;
  color: white;
  padding: 0.6rem 1rem;
  border-radius: 6px;
  text-decoration: none;
  transition: background 0.2s;
}

a.nav-btn:hover {
  background: #0056b3;
}

a.nav-btn.disabled {
  background: #ccc;
  pointer-events: none;
}
.element-invisible {
  position: absolute !important;
  clip: rect(1px 1px 1px 1px);
  clip: rect(1px, 1px, 1px, 1px);
  overflow: hidden;
  height: 1px;
}