css architecture bem utility-first methodology scalability maintainability

CSS architecture patterns: BEM and utility-first approaches

Alex Chang Feb 2026

2 tabs

/* BEM - Block Element Modifier */

/* BLOCK - Independent component */
.card {
  padding: 1rem;
  border: 1px solid #ddd;
  border-radius: 8px;
  background: white;
}

/* ELEMENT - Part of block (uses __) */
.card__header {
  padding-bottom: 0.5rem;
  border-bottom: 1px solid #eee;
}

.card__title {
  margin: 0;
  font-size: 1.25rem;
  font-weight: bold;
}

.card__subtitle {
  margin: 0.25rem 0 0;
  font-size: 0.875rem;
  color: #666;
}

.card__body {
  padding: 1rem 0;
}

.card__footer {
  padding-top: 0.5rem;
  border-top: 1px solid #eee;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.card__image {
  width: 100%;
  height: auto;
  border-radius: 4px;
}

.card__button {
  padding: 0.5rem 1rem;
  background: #3b82f6;
  color: white;
  border: none;
  border-radius: 4px;
  cursor: pointer;
}

/* MODIFIER - Variation of block/element (uses --) */
.card--featured {
  border-color: #3b82f6;
  box-shadow: 0 4px 8px rgba(59, 130, 246, 0.2);
}

.card--compact {
  padding: 0.5rem;
}

.card--horizontal {
  display: flex;
  flex-direction: row;
}

.card__title--large {
  font-size: 1.75rem;
}

.card__button--secondary {
  background: #6b7280;
}

.card__button--disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

/* More BEM examples */
.navigation {
  display: flex;
  background: #1f2937;
}

.navigation__list {
  display: flex;
  list-style: none;
  margin: 0;
  padding: 0;
}

.navigation__item {
  margin: 0;
}

.navigation__link {
  display: block;
  padding: 1rem;
  color: white;
  text-decoration: none;
}

.navigation__link--active {
  background: #3b82f6;
  font-weight: bold;
}

.navigation--vertical .navigation__list {
  flex-direction: column;
}

/* Form with BEM */
.form {
  max-width: 500px;
}

.form__group {
  margin-bottom: 1rem;
}

.form__label {
  display: block;
  margin-bottom: 0.25rem;
  font-weight: 500;
}

.form__input {
  width: 100%;
  padding: 0.5rem;
  border: 1px solid #ddd;
  border-radius: 4px;
}

.form__input--error {
  border-color: #ef4444;
}

.form__error {
  color: #ef4444;
  font-size: 0.875rem;
  margin-top: 0.25rem;
}

.form__submit {
  padding: 0.75rem 1.5rem;
  background: #3b82f6;
  color: white;
  border: none;
  border-radius: 4px;
  cursor: pointer;
}

.form--inline .form__group {
  display: inline-block;
  margin-right: 1rem;
}

<!-- Utility-First CSS Approach -->

<!DOCTYPE html>
<html>
<head>
  <style>
    /* Utility classes - single purpose */

    /* Display */
    .block { display: block; }
    .inline-block { display: inline-block; }
    .flex { display: flex; }
    .grid { display: grid; }
    .hidden { display: none; }

    /* Flexbox utilities */
    .flex-row { flex-direction: row; }
    .flex-col { flex-direction: column; }
    .items-center { align-items: center; }
    .items-start { align-items: flex-start; }
    .justify-center { justify-content: center; }
    .justify-between { justify-content: space-between; }
    .gap-1 { gap: 0.25rem; }
    .gap-2 { gap: 0.5rem; }
    .gap-4 { gap: 1rem; }

    /* Spacing - Padding */
    .p-0 { padding: 0; }
    .p-2 { padding: 0.5rem; }
    .p-4 { padding: 1rem; }
    .p-6 { padding: 1.5rem; }
    .px-4 { padding-left: 1rem; padding-right: 1rem; }
    .py-2 { padding-top: 0.5rem; padding-bottom: 0.5rem; }
    .pt-4 { padding-top: 1rem; }

    /* Spacing - Margin */
    .m-0 { margin: 0; }
    .m-2 { margin: 0.5rem; }
    .m-4 { margin: 1rem; }
    .mx-auto { margin-left: auto; margin-right: auto; }
    .mt-4 { margin-top: 1rem; }
    .mb-2 { margin-bottom: 0.5rem; }

    /* Width */
    .w-full { width: 100%; }
    .w-1\/2 { width: 50%; }
    .w-1\/3 { width: 33.333%; }
    .max-w-lg { max-width: 1024px; }

    /* Typography */
    .text-center { text-align: center; }
    .text-left { text-align: left; }
    .text-sm { font-size: 0.875rem; }
    .text-base { font-size: 1rem; }
    .text-lg { font-size: 1.125rem; }
    .text-xl { font-size: 1.25rem; }
    .font-bold { font-weight: 700; }
    .font-semibold { font-weight: 600; }
    .uppercase { text-transform: uppercase; }

    /* Colors */
    .text-gray-600 { color: #6b7280; }
    .text-blue-600 { color: #3b82f6; }
    .bg-white { background-color: white; }
    .bg-gray-100 { background-color: #f3f4f6; }
    .bg-blue-500 { background-color: #3b82f6; }

    /* Borders */
    .border { border: 1px solid #e5e7eb; }
    .border-2 { border: 2px solid; }
    .border-blue-500 { border-color: #3b82f6; }
    .rounded { border-radius: 0.25rem; }
    .rounded-lg { border-radius: 0.5rem; }
    .rounded-full { border-radius: 9999px; }

    /* Shadows */
    .shadow-sm { box-shadow: 0 1px 2px 0 rgba(0,0,0,0.05); }
    .shadow { box-shadow: 0 1px 3px 0 rgba(0,0,0,0.1); }
    .shadow-lg { box-shadow: 0 10px 15px -3px rgba(0,0,0,0.1); }

    /* Interactive */
    .cursor-pointer { cursor: pointer; }
    .hover\:bg-blue-600:hover { background-color: #2563eb; }
    .hover\:shadow-lg:hover {
      box-shadow: 0 10px 15px -3px rgba(0,0,0,0.1);
    }

    /* Responsive utilities */
    @media (min-width: 768px) {
      .md\:flex { display: flex; }
      .md\:w-1\/2 { width: 50%; }
      .md\:p-8 { padding: 2rem; }
    }

    @media (min-width: 1024px) {
      .lg\:grid-cols-3 { grid-template-columns: repeat(3, 1fr); }
    }
  </style>
</head>
<body class="bg-gray-100">

  <!-- Card using utility classes -->
  <div class="max-w-lg mx-auto mt-4 p-4">
    <div class="bg-white rounded-lg shadow p-6">
      <h2 class="text-xl font-bold mb-2">Card Title</h2>
      <p class="text-gray-600 mb-4">
        This card is built entirely with utility classes.
      </p>
      <div class="flex gap-2">
        <button class="bg-blue-500 text-white px-4 py-2 rounded hover:bg-blue-600 cursor-pointer">
          Primary
        </button>
        <button class="border border-blue-500 text-blue-600 px-4 py-2 rounded hover:bg-blue-50 cursor-pointer">
          Secondary
        </button>
      </div>
    </div>
  </div>

  <!-- Grid layout with utilities -->
  <div class="max-w-lg mx-auto mt-4 p-4 grid gap-4 md:grid-cols-2 lg:grid-cols-3">
    <div class="bg-white p-4 rounded shadow-sm hover:shadow-lg">
      <h3 class="font-semibold mb-2">Item 1</h3>
      <p class="text-sm text-gray-600">Description</p>
    </div>
    <div class="bg-white p-4 rounded shadow-sm hover:shadow-lg">
      <h3 class="font-semibold mb-2">Item 2</h3>
      <p class="text-sm text-gray-600">Description</p>
    </div>
    <div class="bg-white p-4 rounded shadow-sm hover:shadow-lg">
      <h3 class="font-semibold mb-2">Item 3</h3>
      <p class="text-sm text-gray-600">Description</p>
    </div>
  </div>

  <!-- Flexbox layout -->
  <div class="flex items-center justify-between p-4 bg-white border-2 border-blue-500 rounded-lg mx-auto max-w-lg mt-4">
    <div class="flex items-center gap-4">
      <div class="w-12 h-12 bg-blue-500 rounded-full"></div>
      <div>
        <h4 class="font-bold">Username</h4>
        <p class="text-sm text-gray-600">Role</p>
      </div>
    </div>
    <button class="px-4 py-2 bg-blue-500 text-white rounded hover:bg-blue-600">
      Follow
    </button>
  </div>

</body>
</html>

2 files · css, html Explain with highlit

BEM (Block Element Modifier) naming uses .block__element--modifier pattern for scalable CSS. I define blocks as independent components like .card. Elements within blocks use .card__title and .card__body. Modifiers indicate variations with .card--featured or .card__title--large. BEM prevents specificity conflicts and naming collisions. Utility-first CSS uses single-purpose classes like .flex, .p-4, .text-center. I compose utilities for rapid development without writing custom CSS. Frameworks like Tailwind CSS popularized utility-first methodology. The SMACSS methodology organizes CSS into Base, Layout, Module, State, and Theme categories. ITCSS (Inverted Triangle CSS) structures from generic to specific. Understanding architecture patterns improves large-scale CSS maintainability.