CSS

/* Element */
p { color: blue; }

/* Class */
.card { background: #f0f0f0; padding: 20px; }

/* ID */
#header { height: 60px; }

/* Attribute */
input[type="text"] { border: 1px solid gray; }

.container {
  display: flex;
  justify-content: center;  /* horizontal */
  align-items: center;       /* vertical */
  gap: 16px;
  flex-wrap: wrap;
}

.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 20px;
}

@media (max-width: 768px) {
  .grid {
    grid-template-columns: 1fr;
  }
  .sidebar { display: none; }
}