HTML

<nav class="site-nav">
  <div class="brand">Company Name</div>
  <button class="menu-toggle" aria-label="Toggle menu">☰</button>
  <ul class="nav-menu">
    <li><a href="#">Home</a></li>
    <li><a href="#">Products</a></li>
    <li><a href="#">Services</a></li>
    <li><a href="#">About Us</a></li>
    <li><a href="#">Contact</a></li>
  </ul>
</nav>

Mobile-First CSS

/* Base (Mobile) Styles */
.site-nav {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  align-items: center;
  padding: 1rem;
  background-color: #f8f9fa;
}

.brand {
  font-size: 1.5rem;
  font-weight: bold;
}

.menu-toggle {
  display: block; /* Visible on mobile */
  background: none;
  border: 1px solid #ddd;
  padding: 0.5rem;
  font-size: 1.25rem;
  cursor: pointer;
}

.nav-menu {
  width: 100%; /* Full width on mobile */
  padding: 0;
  margin: 1rem 0 0 0;
  list-style: none;
  display: none; /* Hidden by default */
}

.nav-menu.active {
  display: block; /* Show when active */
}

.nav-menu li {
  margin-bottom: 0.5rem;
}

.nav-menu a {
  display: block;
  padding: 0.5rem 0;
  text-decoration: none;
  color: #333;
}

/* Tablet and Desktop Enhancements */
@media (min-width: 768px) {
  .menu-toggle {
    display: none; /* Hide toggle on larger screens */
  }
  
  .nav-menu {
    display: flex; /* Always visible as a row */
    width: auto; /* Auto width */
    margin-top: 0; /* Remove margin */
  }
  
  .nav-menu li {
    margin: 0 0 0 1.5rem; /* Margin between items */
  }
}

JavaScript for Toggle

document.querySelector('.menu-toggle').addEventListener('click', function() {
  document.querySelector('.nav-menu').classList.toggle('active');
});

HTML

<div class="product-grid">
  <div class="product-card">
    <img src="product1.jpg" alt="Product 1">
    <h3>Product Name</h3>
    <p class="price">$19.99</p>
    <button class="add-to-cart">Add to Cart</button>
  </div>
  <!-- More product cards... -->
</div>

Mobile-First CSS

/* Base (Mobile) Styles */
.product-grid {
  display: grid;
  grid-template-columns: 1fr; /* Single column on mobile */
  gap: 1rem;
  padding: 1rem;
}

.product-card {
  border: 1px solid #eee;
  border-radius: 8px;
  padding: 1rem;
  display: flex;
  flex-direction: column;
}

.product-card img {
  width: 100%;
  height: auto;
  object-fit: cover;
  border-radius: 4px;
  margin-bottom: 0.75rem;
}

.product-card h3 {
  margin: 0 0 0.5rem 0;
  font-size: 1.1rem;
}

.price {
  font-weight: bold;
  color: #e63946;
  margin-bottom: 0.75rem;
}

.add-to-cart {
  margin-top: auto; /* Push to bottom */
  padding: 0.5rem;
  background-color: #1d3557;
  color: white;
  border: none;
  border-radius: 4px;
  cursor: pointer;
}

/* Tablet Enhancement */
@media (min-width: 576px) {
  .product-grid {
    grid-template-columns: repeat(2, 1fr); /* 2 columns on tablet */
  }
}

/* Desktop Enhancement */
@media (min-width: 992px) {
  .product-grid {
    grid-template-columns: repeat(3, 1fr); /* 3 columns on smaller desktops */
    gap: 1.5rem;
    padding: 1.5rem;
  }
  
  .product-card {
    transition: transform 0.3s ease, box-shadow 0.3s ease;
  }
  
  .product-card:hover {
    transform: translateY(-5px);
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.1);
  }
}

/* Large Desktop Enhancement */
@media (min-width: 1200px) {
  .product-grid {
    grid-template-columns: repeat(4, 1fr); /* 4 columns on large desktops */
    max-width: 1400px;
    margin: 0 auto;
  }
}