Complete reference for HTML5 semantic elements, CSS3 layout (Flexbox, Grid), animations, forms, and responsive design with copy-paste examples.
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Save as .html file and open in any modern browser.
Production-ready HTML5 boilerplate with SEO and accessibility.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="description" content="Page description for search engines (150-160 chars)">
<meta name="keywords" content="keyword1, keyword2, keyword3">
<meta name="author" content="MyWebUniversity.com">
<meta name="robots" content="index, follow">
<!-- Open Graph -->
<meta property="og:title" content="Page Title | Site Name">
<meta property="og:description" content="Description for social sharing">
<meta property="og:type" content="website">
<meta property="og:url" content="https://example.com/page">
<meta property="og:image" content="https://example.com/og-image.jpg">
<!-- Twitter Card -->
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:title" content="Page Title">
<meta name="twitter:description" content="Description">
<link rel="canonical" href="https://example.com/page">
<title>Page Title | Site Name</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<a href="#main" class="skip-link">Skip to main content</a>
<header role="banner">
<nav aria-label="Primary navigation">
<ul>
<li><a href="/" aria-current="page">Home</a></li>
<li><a href="/courses">Courses</a></li>
<li><a href="/about">About</a></li>
</ul>
</nav>
</header>
<main id="main" tabindex="-1">
<article>
<header>
<h1>Main Page Heading</h1>
<time datetime="2026-06-25">June 25, 2026</time>
</header>
<p>Content goes here…</p>
</article>
</main>
<aside aria-label="Related content">
<h2>Related Topics</h2>
</aside>
<footer role="contentinfo">
<p>© 2026 MyWebUniversity.com</p>
</footer>
<script src="app.js" defer></script>
</body>
</html>CSS custom properties for theming and dark mode.
/* style.css — Dark theme with CSS variables and responsive grid */
:root {
/* Light theme defaults */
--bg: #ffffff;
--surface: #f8fafc;
--border: #e2e8f0;
--text: #0f172a;
--muted: #64748b;
--accent: #2563eb;
--radius: 8px;
}
@media (prefers-color-scheme: dark) {
:root {
--bg: #0f172a;
--surface: #1e293b;
--border: #334155;
--text: #e2e8f0;
--muted: #94a3b8;
--accent: #60a5fa;
}
}
/* Manual dark mode override */
[data-theme="dark"] {
--bg: #0f172a;
--surface: #1e293b;
--border: #334155;
--text: #e2e8f0;
--accent: #60a5fa;
}
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
body { background: var(--bg); color: var(--text); font-family: system-ui, sans-serif; line-height: 1.6; }
.card { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; }
.btn { padding: 10px 24px; background: var(--accent); color: #fff; border: none; border-radius: var(--radius); cursor: pointer; font-size: 1rem; }
/* Toggle button in JavaScript */
/* document.querySelector('[data-theme-toggle]')
.addEventListener('click', () =>
document.documentElement.dataset.theme =
document.documentElement.dataset.theme === 'dark' ? 'light' : 'dark'); */CSS Grid layout that adapts to any screen size.
/* Responsive Grid — no media queries needed for the grid itself */
.page {
display: grid;
grid-template-areas:
"header"
"main"
"footer";
grid-template-rows: auto 1fr auto;
min-height: 100vh;
}
@media (min-width: 768px) {
.page {
grid-template-areas:
"header header"
"sidebar main"
"footer footer";
grid-template-columns: 240px 1fr;
}
}
/* Auto-responsive card grid (no media queries!) */
.card-grid {
display: grid;
grid-template-columns: repeat(auto-fit, minmax(min(100%, 280px), 1fr));
gap: 1.5rem;
padding: 1.5rem;
}
/* Dashboard widgets */
.dashboard {
display: grid;
grid-template-columns: repeat(12, 1fr);
gap: 16px;
}
.widget { grid-column: span 12; }
.widget-half { grid-column: span 6; }
.widget-third { grid-column: span 4; }
.widget-quarter{ grid-column: span 3; }
@media (max-width: 900px) {
.widget-half, .widget-third, .widget-quarter { grid-column: span 12; }
}Smooth transitions and keyframe animations.
/* CSS Animations & Transitions */
/* Transition — smooth state changes */
.btn {
background: #2563eb;
color: #fff;
padding: 10px 24px;
border-radius: 8px;
transition: background 0.2s ease, transform 0.1s ease, box-shadow 0.2s ease;
}
.btn:hover { background: #1d4ed8; transform: translateY(-2px); box-shadow: 0 4px 12px rgba(37,99,235,.4); }
.btn:active { transform: translateY(0); box-shadow: none; }
/* Fade in on load */
@keyframes fadeInUp {
from { opacity: 0; transform: translateY(20px); }
to { opacity: 1; transform: translateY(0); }
}
.card { animation: fadeInUp 0.4s ease forwards; }
.card:nth-child(2) { animation-delay: 0.1s; }
.card:nth-child(3) { animation-delay: 0.2s; }
/* Spinner */
@keyframes spin { to { transform: rotate(360deg); } }
.spinner {
width: 32px; height: 32px;
border: 3px solid #334155;
border-top-color: #60a5fa;
border-radius: 50%;
animation: spin 0.8s linear infinite;
}
/* Pulse */
@keyframes pulse {
0%, 100% { opacity: 1; }
50% { opacity: 0.5; }
}
.skeleton { background: #1e293b; border-radius: 4px; animation: pulse 1.5s ease infinite; }
/* Scroll-triggered (with JS IntersectionObserver) */
.reveal { opacity: 0; transform: translateY(30px); transition: opacity 0.6s ease, transform 0.6s ease; }
.reveal.visible { opacity: 1; transform: translateY(0); }
/*
const observer = new IntersectionObserver(entries =>
entries.forEach(e => e.isIntersecting && e.target.classList.add('visible')));
document.querySelectorAll('.reveal').forEach(el => observer.observe(el));
*/Semantic, accessible HTML form with validation and ARIA.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Enrollment Form | MyWebUniversity</title>
<style>
:root { --accent:#22d3ee; --bg:#0f172a; --surface:#1e293b; --border:#334155; --text:#e5e7eb; }
*, *::before, *::after { box-sizing: border-box; }
body { font-family: system-ui; background: var(--bg); color: var(--text); padding: 40px 20px; }
.form { max-width: 480px; margin: 0 auto; background: var(--surface);
border: 1px solid var(--border); border-radius: 10px; padding: 28px; }
label { display: block; font-size: .9em; font-weight: 600; margin-bottom: 5px; color: var(--accent); }
input, select { width: 100%; padding: 10px 12px; background: var(--bg); border: 1px solid var(--border);
border-radius: 6px; color: var(--text); font-size: 1rem; margin-bottom: 16px; }
input:focus, select:focus { outline: none; border-color: var(--accent); }
input:invalid:not(:placeholder-shown) { border-color: #f43f5e; }
.error { color: #f43f5e; font-size: .85em; margin-top: -12px; margin-bottom: 12px; display: none; }
input:invalid:not(:placeholder-shown) + .error { display: block; }
.btn { width: 100%; padding: 12px; background: var(--accent); color: #0f172a;
border: none; border-radius: 6px; font-size: 1rem; font-weight: 700; cursor: pointer; }
.btn:hover { opacity: .9; }
.required::after { content: " *"; color: #f43f5e; }
</style>
</head>
<body>
<form class="form" novalidate id="enrollForm" aria-labelledby="formTitle">
<h2 id="formTitle">Enroll in a Course</h2>
<p role="note" style="color:#9ca3af;font-size:.9em;margin-bottom:20px">
Fields marked <span style="color:#f43f5e">*</span> are required.
</p>
<label for="name" class="required">Full Name</label>
<input type="text" id="name" name="name" required minlength="2"
placeholder="Alice Smith" autocomplete="name" aria-required="true">
<div class="error" id="name-error" role="alert">Please enter your full name (2+ characters).</div>
<label for="email" class="required">Email Address</label>
<input type="email" id="email" name="email" required
placeholder="alice@example.com" autocomplete="email" aria-required="true">
<div class="error" id="email-error" role="alert">Please enter a valid email address.</div>
<label for="course" class="required">Select Course</label>
<select id="course" name="course" required aria-required="true">
<option value="" disabled selected>-- Choose a course --</option>
<option value="js">Chapter 30 — JavaScript</option>
<option value="html">Chapter 31 — HTML & CSS</option>
<option value="py">Chapter 1 — Python</option>
</select>
<button type="submit" class="btn" aria-label="Submit enrollment form">
Enroll Now
</button>
</form>
<script>
document.getElementById('enrollForm').addEventListener('submit', e => {
e.preventDefault();
if (e.target.checkValidity()) alert('Enrolled! Thank you.');
else e.target.reportValidity();
});
</script>
</body>
</html>