HTML & CSS

Modern HTML5 and CSS3 for semantic web structure and responsive styling.

When to Use

• Semantic Web Structure & Accessibility: Crafting accessible, WCAG-compliant, SEO-optimized web documents with HTML5 elements.
• Modern Responsive Layouts (CSS Grid & Flexbox): Designing fluid, adaptable screen layouts across mobile, tablet, desktop, and TV viewports.
• Component-Driven Theming (CSS Custom Properties): Implementing zero-runtime dark/light mode themes and design tokens.
• Modern CSS Features (Container Queries & :has()): Styling components based on their immediate container width and parent-child states.

Quick Start

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Page Title</title>
    <link rel="stylesheet" href="styles.css" />
  </head>
  <body>
    <main class="container">
      <h1>Hello World</h1>
    </main>
  </body>
</html>

Core Concepts

Semantic HTML5 & Accessible Landmark Roles

Replaces generic <div> soup with structural elements that assistive technologies understand:

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Enterprise Portal</title>
  </head>
  <body>
    <header role="banner">
      <nav aria-label="Main Navigation">
        <ul>
          <li><a href="/dashboard">Dashboard</a></li>
        </ul>
      </nav>
    </header>
    <main id="main-content">
      <article>
        <h1>System Architecture 2026</h1>
        <p>Modern web applications prioritize semantic accessibility.</p>
      </article>
    </main>
    <footer role="contentinfo">
      <p>&copy; 2026 Enterprise Corp.</p>
    </footer>
  </body>
</html>

Modern CSS Layout (Grid & Subgrid)

Builds responsive multi-column layouts without external CSS frameworks:

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 1.5rem;
}

.card {
  display: grid;
  grid-template-rows: subgrid;
  grid-row: span 3;
  padding: 1.5rem;
  background-color: var(--color-surface);
  border-radius: 0.75rem;
}

Container Queries & CSS Custom Properties

Adapts styling based on container container width rather than the global browser viewport:

:root {
  --color-primary: #4f46e5;
  --color-surface: #ffffff;
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-surface: #0f172a;
  }
}

.card-container {
  container-type: inline-size;
}

@container (min-width: 450px) {
  .card-content {
    display: flex;
    gap: 1rem;
  }
}

Common Patterns

Centered Responsive Component Layout

Problem: Vertically and horizontally centering responsive card elements across varying viewport dimensions.

Solution:

/* Center content */
.center {
  display: flex;
  justify-content: center;
  align-items: center;
}

/* Navigation bar */
.navbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 1rem;
}

/* Card grid with wrap */
.card-container {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
}

.card {
  flex: 1 1 300px;
}

CSS Grid

Problem: Constructing multi-dimensional grid layouts and responsive page containers without fragile positioning hacks.

Solution:

/* Responsive grid */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 1.5rem;
}

/* Page layout */
.layout {
  display: grid;
  grid-template-areas:
    "header header"
    "sidebar main"
    "footer footer";
  grid-template-columns: 250px 1fr;
  grid-template-rows: auto 1fr auto;
  min-height: 100vh;
}

.header {
  grid-area: header;
}
.sidebar {
  grid-area: sidebar;
}
.main {
  grid-area: main;
}
.footer {
  grid-area: footer;
}

Responsive Design

Problem: Maintaining layout proportion, typography readability, and container responsiveness across disparate screen viewports.

Solution:

/* Mobile-first approach */
.container {
  padding: 1rem;
}

@media (min-width: 768px) {
  .container {
    padding: 2rem;
    max-width: 1200px;
    margin: 0 auto;
  }
}

/* Container queries */
.card-container {
  container-type: inline-size;
}

@container (min-width: 400px) {
  .card {
    display: flex;
    gap: 1rem;
  }
}

Best Practices

Do:

• Always Include Viewport Meta Tag: Ensure <meta name="viewport" content="width=device-width, initial-scale=1.0"> is present.
• Use Native CSS Nesting and :has(): Eliminate Sass build steps by leveraging native browser CSS nesting and relational selectors.
• Design with Accessibility in Mind: Ensure color contrast ratios meet WCAG AA standards (4.5:1) and all images have descriptive alt tags.
• Implement Fluid Typography with clamp(): Use font-size: clamp(1rem, 2.5vw, 2rem); for seamless responsive scaling.

Don't:

• Use !important to resolve specificity issues: Structure cascade layers with @layer instead of overriding specificity brute-force.
• Disable focus outlines without alternatives: Never write outline: none; without providing an accessible :focus-visible ring.
• Use non-semantic elements for interactive buttons: Never use <div onclick="...">; always use <button> to ensure keyboard accessibility.

Troubleshooting

| Error | Cause | Solution | | ----------------------- | ------------------- | ------------------------- | | Layout overflow | Fixed widths | Use percentage or min/max | | Flex items not wrapping | Missing flex-wrap | Add flex-wrap: wrap | | Grid gaps not showing | Old browser | Check browser support |

References

• MDN Web Docs
• CSS-Tricks