HTML Master Course
Master HTML From Zero to Expert
Chandrakanta Behera
This course includes:
What you'll learn
Master hTML Foundations
Master document Structure
Master text & Semantics
Master links & Navigation
Master images & Figures
Master tables
Master forms
Master aRIA & Accessibility
Course content
8 sections
42 lectures • 0 min total length
Inside this course
8
Modules
42
Lessons
1 hour
Content
Lifetime
Access
Master HTML From Zero to Expert HTML Master Course is a complete, hands-on program covering html foundations, document structure, text & semantics and more — 42 lessons across 8 structured modules, with real projects you can ship from day one.
What you will build
Master HTML Foundations
Master document Structure
Master text & Semantics
Master links & Navigation
Master images & Figures
Master tables
Who this is for
Absolute beginners learning HTML from zero
Educators who want a complete interactive reference
Developers filling gaps in semantic, modern HTML
Curriculum Chapters (24 Verified Modules)
Hands-On Deep-Dive↓HTML Foundations
HTML (HyperText Markup Language) is the standard markup language for creating web pages. It describes the structure of content — not its appearance. C...
Document Structure
Every HTML document follows a specific skeleton. Understanding this skeleton is essential before writing a single tag.
Text & Semantics
Semantic HTML means using elements that convey the meaning of content, not just its appearance. This benefits SEO, accessibility, and maintainability....
Lists
Tables are for tabular data — rows and columns of related information. Never use them for page layout.
Links & Navigation
Images & Figures
Tables
Forms
Forms are the primary way users send data to a server. Every form element needs afor accessibility.
Input Types
HTML5 introduced many semantic input types that trigger appropriate keyboards on mobile and enable built-in validation.
Form Validation
SVG is an XML-based vector format that can be embedded directly in HTML. SVGs are resolution-independent, scriptable, and styleable with CSS.
Audio & Video
Responsive Images
iFrame & Embedded Content
SVG — Scalable Vector Graphics
Canvas 2D API
Theelement provides a pixel-based drawing surface. Drawing is done entirely via JavaScript using the Canvas 2D API.
WebGL
WebGL provides a low-level GPU-accelerated 3D graphics API inside. It is based on OpenGL ES and uses GLSL shaders.
HTML5 Semantic Elements
HTML5 introduced landmark elements that describe the purpose of page regions, enabling better accessibility, SEO, and browser features.
ARIA & Accessibility
ARIA (Accessible Rich Internet Applications) attributes extend HTML semantics for assistive technologies. The first rule of ARIA: don't use ARIA if na...
SEO & Meta Tags
This is aelement — a native modal with a backdrop, focus trap, and Escape-to-close. No library needed.
Modern HTML Elements
Web Components
Web Components are a suite of standards for building reusable, encapsulated custom HTML elements.
Performance & Loading
Script downloads in parallel, executes after HTML is fully parsed. Order preserved. Use for most scripts.
HTML APIs
Write semantic HTML before adding CSS or JS. Your HTML alone should make sense.
Best Practices & Checklist
Write semantic HTML before adding CSS or JS. Your HTML alone should make sense.
Comprehensive Interactive Syllabus Below
This course includes an extensive interactive living syllabus with real-world project specifications, architecture diagrams, and comprehensive lessons.
Instructor
Chandrakanta Behera
Founder & lead instructor at CodeBitByBit — building and teaching real-world AI, mobile, and full-stack engineering.
chandrakantabehera.dev ↗3
Students
1
Courses
0.0
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Interactive Curriculum & Hands-On Deep-Dive
Explore the entire verified syllabus, hands-on modules, project deliverables, and real-world implementation specs for HTML Master Course.
HTML Foundations
HTML (HyperText Markup Language) is the standard markup language for creating web pages. It describes the structure of content — not its appearance. CSS handles styling; JavaScript handles behaviour.
Markup
Elements wrap content in angle-bracket tags to give it meaning and structure.
DOM Tree
The browser parses HTML into a Document Object Model — a live tree of nodes.
HyperText
Links connect documents across the web, forming the "hyper" in HyperText.
Standards
HTML is maintained by WHATWG as a living standard; no version numbers anymore.
<!-- Opening tag Content Closing tag -->
<p class="intro">Hello, World!</p>
<!-- Void element — self-closing, no content -->
<img src="photo.jpg" alt="A photo" />
<!-- Attributes: name="value" pairs on the opening tag -->
<a href="https://example.com" target="_blank" rel="noopener">
Visit Example
</a>Document Structure
Every HTML document follows a specific skeleton. Understanding this skeleton is essential before writing a single tag.
<!-- DOCTYPE declaration: tells browser this is HTML5 -->
<!DOCTYPE html>
<!-- Root element with language attribute -->
<html lang="en">
<head>
<!-- Character encoding — MUST be first in <head> -->
<meta charset="UTF-8" />
<!-- Responsive viewport -->
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<!-- Page title shown in tab and search results -->
<title>My Page Title</title>
<!-- Meta description for SEO -->
<meta name="description" content="Page description here" />
<!-- External CSS -->
<link rel="stylesheet" href="styles.css" />
<!-- Inline CSS -->
<style> body { margin: 0; } </style>
<!-- Favicon -->
<link rel="icon" href="favicon.ico" />
<!-- Preload critical resource -->
<link rel="preload" href="font.woff2" as="font" crossorigin />
</head>
<body>
<!-- All visible content goes here -->
<h1>Hello World</h1>
<!-- Scripts at bottom for performance -->
<script src="app.js" defer></script>
</body>
</html>Text & Semantics
Semantic HTML means using elements that convey the meaning of content, not just its appearance. This benefits SEO, accessibility, and maintainability.
<h1>Page Title — only ONE per page</h1>
<h2>Major Section</h2>
<h3>Sub-section</h3>
<h4>Sub-sub-section</h4>
<h5>Rarely needed</h5>
<h6>Very rarely needed</h6><p>A paragraph of text.</p>
<!-- Block quotation with citation -->
<blockquote cite="https://source.com">
<p>The best way to predict the future is to invent it.</p>
<footer>— <cite>Alan Kay</cite></footer>
</blockquote>
<!-- Address contact info for author/owner -->
<address>
Written by <a href="mailto:[email protected]">Author Name</a>
</address>
<!-- Thematic break -->
<hr />
<!-- Generic block container -->
<div class="wrapper">...</div>Lists
Tables are for tabular data — rows and columns of related information. Never use them for page layout.
<!-- Unordered list -->
<ul>
<li>Apples</li>
<li>Bananas</li>
<li>Nested list:
<ul><li>Sub-item</li></ul>
</li>
</ul>
<!-- Ordered list with custom start and type -->
<ol start="3" type="A" reversed>
<li>Step three</li>
<li value="10">Jump to item 10</li>
</ol>
<!-- Description list — terms and definitions -->
<dl>
<dt>HTML</dt>
<dd>HyperText Markup Language — structure of the web.</dd>
<dt>CSS</dt>
<dd>Cascading Style Sheets — appearance of the web.</dd>
</dl>
<!-- Menu (list of commands — spec usage) -->
<menu>
<li><button>Cut</button></li>
<li><button>Copy</button></li>
</menu>Links & Navigation
<!-- Standard hyperlink -->
<a href="https://example.com">External link</a>
<!-- Open in new tab safely -->
<a href="https://example.com"
target="_blank"
rel="noopener noreferrer">New tab</a>
<!-- In-page anchor -->
<a href="#section-id">Jump to section</a>
<!-- Email link -->
<a href="mailto:[email protected]?subject=Hi&body=Hello">Send email</a>
<!-- Phone link -->
<a href="tel:+15551234567">Call us</a>
<!-- SMS link -->
<a href="sms:+15551234567?body=Hello">Text us</a>
<!-- Download link -->
<a href="/files/report.pdf" download="Annual-Report.pdf">Download PDF</a>
<!-- Navigation landmark -->
<nav aria-label="Main navigation">
<ul>
<li><a href="/" aria-current="page">Home</a></li>
<li><a href="/about">About</a></li>
</ul>
</nav>Images & Figures
<!-- Basic image — alt is MANDATORY for accessibility -->
<img
src="photo.jpg"
alt="A golden retriever sitting in a park"
width="800"
height="600"
loading="lazy"
decoding="async"
/>
<!-- Figure with caption -->
<figure>
<img src="chart.png" alt="Bar chart showing Q3 revenue growth" />
<figcaption>Figure 1: Q3 2024 Revenue Growth by Region</figcaption>
</figure>
<!-- Decorative image (empty alt = skip by screen reader) -->
<img src="divider.svg" alt="" role="presentation" />
<!-- Image map with clickable regions -->
<img src="map.jpg" alt="World map" usemap="#worldmap" />
<map name="worldmap">
<area shape="rect" coords="0,0,200,100" href="/north-america" alt="North America" />
<area shape="circle" coords="300,150,50" href="/europe" alt="Europe" />
</map>Tables
<table>
<!-- Caption: acts as the table's heading -->
<caption>Monthly Sales Data — Q1 2025</caption>
<!-- Column group styling hooks -->
<colgroup>
<col span="1" style="background:#f0f"/>
<col span="3" />
</colgroup>
<!-- Table head -->
<thead>
<tr>
<th scope="col">Month</th>
<th scope="col">Revenue</th>
<th scope="col">Units</th>
</tr>
</thead>
<!-- Table body -->
<tbody>
<tr>
<th scope="row">January</th>
<td>$12,400</td>
<td>248</td>
</tr>
<tr>
<!-- colspan / rowspan for merged cells -->
<td colspan="2">Merged cell</td>
<td rowspan="2">Spans 2 rows</td>
</tr>
</tbody>
<!-- Table foot — totals / summaries -->
<tfoot>
<tr>
<th scope="row">Total</th>
<td>$37,200</td>
<td>744</td>
</tr>
</tfoot>
</table>Forms
Forms are the primary way users send data to a server. Every form element needs a
Input Types
HTML5 introduced many semantic input types that trigger appropriate keyboards on mobile and enable built-in validation.
<input type="text" /> <!-- default -->
<input type="email" /> <!-- validates email format -->
<input type="password" /> <!-- masks input -->
<input type="number" /> <!-- numeric keyboard on mobile -->
<input type="tel" /> <!-- phone keyboard on mobile -->
<input type="url" /> <!-- validates URL format -->
<input type="search" /> <!-- search field with clear button -->
<input type="date" /> <!-- date picker -->
<input type="time" /> <!-- time picker -->
<input type="datetime-local" />
<input type="month" />
<input type="week" />
<input type="color" /> <!-- colour picker -->
<input type="range" min="0" max="100" step="1" />
<input type="checkbox" />
<input type="radio" name="group" />
<input type="file" accept="image/*" multiple />
<input type="hidden" value="token" />
<input type="submit" value="Send" />
<input type="reset" />
<input type="button" value="Click" />
<input type="image" src="btn.png" alt="Submit" />Form Validation
SVG is an XML-based vector format that can be embedded directly in HTML. SVGs are resolution-independent, scriptable, and styleable with CSS.
<input type="text"
required <!-- Field must be filled -->
minlength="3" <!-- Min char count -->
maxlength="60" <!-- Max char count -->
pattern="[A-Za-zs]+" <!-- Regex pattern -->
title="Letters only please" <!-- Error tooltip hint -->
/>
<input type="number"
min="1"
max="100"
step="5"
/>
<!-- Custom error messages via JavaScript -->
<script>
const input = document.querySelector('input');
input.addEventListener('invalid', (e) => {
e.target.setCustomValidity('Please enter your full name.');
});
input.addEventListener('input', (e) => {
e.target.setCustomValidity('');
});
</script>input:valid { border-color: green; }
input:invalid { border-color: red; }
input:required { outline: 2px solid gold; }
input:optional { }
input:in-range { }
input:out-of-range { }
input:placeholder-shown { }
:user-valid { } /* Only shows after user interaction */
:user-invalid { } /* Better UX than :invalid */Audio & Video
<video
src="movie.mp4"
width="800"
height="450"
controls <!-- Show browser controls -->
autoplay <!-- Auto-play (muted required) -->
muted <!-- Required for autoplay -->
loop <!-- Loop playback -->
playsinline <!-- No full-screen on iOS -->
preload="metadata" <!-- none | metadata | auto -->
poster="thumbnail.jpg" <!-- Shown before play -->
crossorigin="anonymous"
>
<!-- Multiple sources for browser support -->
<source src="movie.av1.mp4" type="video/mp4; codecs=av01"/>
<source src="movie.h264.mp4" type="video/mp4"/>
<source src="movie.webm" type="video/webm"/>
<!-- Subtitles / closed captions -->
<track kind="subtitles" src="subs.en.vtt" srclang="en" label="English" default/>
<track kind="captions" src="cc.en.vtt" srclang="en" label="CC English"/>
<track kind="descriptions" src="desc.vtt" srclang="en"/>
<track kind="chapters" src="chapters.vtt" srclang="en"/>
<track kind="metadata" src="data.vtt"/>
Your browser does not support the video element.
</video>
<!-- Audio -->
<audio controls preload="none">
<source src="audio.ogg" type="audio/ogg"/>
<source src="audio.mp3" type="audio/mpeg"/>
<source src="audio.opus" type="audio/ogg; codecs=opus"/>
</audio>Responsive Images
<!-- Art direction: different crops per breakpoint -->
<picture>
<!-- WebP format with art direction -->
<source
media="(max-width: 600px)"
srcset="hero-mobile.webp"
type="image/webp"
/>
<source
media="(min-width: 601px)"
srcset="hero-desktop.webp"
type="image/webp"
/>
<!-- Fallback img -->
<img src="hero.jpg" alt="Hero image" />
</picture>
<!-- Density / resolution switching -->
<img
src="logo.png"
srcset="logo.png 1x, [email protected] 2x, [email protected] 3x"
alt="Company Logo"
/>
<!-- Width-based srcset with sizes hint -->
<img
src="photo-800.jpg"
srcset="
photo-400.jpg 400w,
photo-800.jpg 800w,
photo-1200.jpg 1200w,
photo-1600.jpg 1600w
"
sizes="
(max-width: 600px) 100vw,
(max-width: 1200px) 50vw,
800px
"
alt="Responsive photo"
loading="lazy"
/>iFrame & Embedded Content
<!-- Basic iframe -->
<iframe
src="https://example.com"
title="Example page" <!-- REQUIRED for accessibility -->
width="800"
height="500"
loading="lazy"
sandbox="allow-scripts allow-same-origin"
referrerpolicy="no-referrer"
allowfullscreen
></iframe>
<!-- YouTube embed -->
<iframe
src="https://www.youtube.com/embed/VIDEO_ID?rel=0"
title="YouTube video player"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope"
allowfullscreen
></iframe>
<!-- sandbox values -->
<!-- allow-scripts — run JS -->
<!-- allow-same-origin — treat as same origin -->
<!-- allow-forms — submit forms -->
<!-- allow-popups — open new windows -->
<!-- allow-top-navigation — navigate parent -->
<!-- allow-downloads — trigger file downloads -->
<!-- Object (legacy) -->
<object data="file.pdf" type="application/pdf" width="600" height="400">
<p>PDF not supported. <a href="file.pdf">Download it.</a></p>
</object>SVG — Scalable Vector Graphics
<svg
xmlns="http://www.w3.org/2000/svg"
viewBox="0 0 200 200"
width="200" height="200"
role="img"
aria-label="Geometric shapes demo"
>
<title>Geometric shapes demo</title>
<!-- Basic shapes -->
<circle cx="50" cy="50" r="40" fill="#4fc3f7" />
<rect x="110" y="10" width="80" height="80" fill="#e8c547" rx="8" />
<ellipse cx="50" cy="150" rx="40" ry="25" fill="#f06292" />
<line x1="110" y1="110" x2="190" y2="190" stroke="white" stroke-width="3" />
<polygon points="150,110 190,190 110,190" fill="#4ade80" />
<polyline points="0,200 30,150 60,180 90,120" stroke="orange" fill="none" />
<!-- Path — the most powerful shape -->
<path d="M10,80 Q95,10 180,80 T350,80" stroke="white" fill="none" />
<!-- Text -->
<text x="100" y="195" text-anchor="middle" fill="white" font-size="12">SVG</text>
<!-- Gradient definition + usage -->
<defs>
<linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="0%">
<stop offset="0%" stop-color="#e8c547"/>
<stop offset="100%" stop-color="#f06292"/>
</linearGradient>
<filter id="blur"><feGaussianBlur stdDeviation="3"/></filter>
<clipPath id="clip"><circle cx="100" cy="100" r="50"/></clipPath>
</defs>
<!-- SVG Animation -->
<circle cx="100" cy="100" r="20" fill="url(#grad)">
<animate attributeName="r" values="20;30;20" dur="2s" repeatCount="indefinite"/>
</circle>
</svg>Canvas 2D API
The
<canvas id="myCanvas" width="600" height="400"
role="img" aria-label="Animated wave chart">
Canvas not supported. <!-- Fallback -->
</canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// Fill & stroke
ctx.fillStyle = '#e8c547';
ctx.fillRect(10, 10, 100, 50);
// Path drawing
ctx.beginPath();
ctx.moveTo(200, 50);
ctx.lineTo(300, 150);
ctx.arcTo(350, 150, 350, 100, 30);
ctx.closePath();
ctx.strokeStyle = '#4fc3f7';
ctx.lineWidth = 3;
ctx.stroke();
// Gradient
const grad = ctx.createLinearGradient(0,0,600,0);
grad.addColorStop(0, '#e8c547');
grad.addColorStop(1, '#f06292');
ctx.fillStyle = grad;
ctx.fillRect(0, 200, 600, 20);
// Text
ctx.font = 'bold 24px Syne';
ctx.fillStyle = '#fff';
ctx.fillText('Canvas 2D', 10, 250);
// Image drawing
const img = new Image();
img.src = 'photo.jpg';
img.onload = () => ctx.drawImage(img, 400, 0, 200, 150);
// Transform
ctx.save();
ctx.translate(100, 300);
ctx.rotate(Math.PI / 4);
ctx.fillRect(-25, -25, 50, 50);
ctx.restore();
// Animation loop
let frame = 0;
function animate() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
frame++;
// draw frame...
requestAnimationFrame(animate);
}
animate();
</script>WebGL
WebGL provides a low-level GPU-accelerated 3D graphics API inside
<canvas id="glCanvas" width="640" height="480"></canvas>
<script>
const gl = canvas.getContext('webgl2') // WebGL 2
?? canvas.getContext('webgl'); // fallback
// Vertex Shader (runs per vertex on GPU)
const vsSource = `
attribute vec4 aPosition;
uniform mat4 uModelViewProjection;
void main() {
gl_Position = uModelViewProjection * aPosition;
}
`;
// Fragment Shader (runs per pixel on GPU)
const fsSource = `
precision mediump float;
uniform vec4 uColor;
void main() {
gl_FragColor = uColor;
}
`;
// Compile shader
function compileShader(gl, type, source) {
const shader = gl.createShader(type);
gl.shaderSource(shader, source);
gl.compileShader(shader);
return shader;
}
// Buffer upload, link program, draw call…
gl.clearColor(0, 0, 0, 1);
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
gl.drawArrays(gl.TRIANGLES, 0, 3);
// Key WebGL concepts:
// VBO — Vertex Buffer Object (geometry on GPU)
// VAO — Vertex Array Object (attribute layout)
// EBO — Element Buffer Object (index buffer)
// FBO — Framebuffer Object (off-screen render)
// UBO — Uniform Buffer Object (WebGL2)
// SSBO — Shader Storage Buffer (WebGL2 ext)
</script>HTML5 Semantic Elements
HTML5 introduced landmark elements that describe the purpose of page regions, enabling better accessibility, SEO, and browser features.
<body>
<!-- ❶ Site header -->
<header>
<nav aria-label="Primary">…</nav>
</header>
<!-- ❷ Skip link for keyboard users -->
<a href="#main-content" class="skip-link">Skip to main content</a>
<!-- ❸ Main content landmark -->
<main id="main-content">
<!-- Article: self-contained composable unit -->
<article>
<header>
<h1>Article Title</h1>
<p>By <address><a href="mailto:[email protected]">Author</a></address></p>
<time datetime="2025-06-01">June 1, 2025</time>
</header>
<!-- Section: thematic grouping inside article -->
<section aria-labelledby="intro-heading">
<h2 id="intro-heading">Introduction</h2>
<p>Content…</p>
</section>
<footer><p>Published by…</p></footer>
</article>
<!-- Aside: tangentially related content -->
<aside aria-label="Related articles">
<h2>Also Read</h2>
…
</aside>
</main>
<!-- ❹ Site footer -->
<footer>
<nav aria-label="Footer navigation">…</nav>
<p>© 2025 My Company</p>
</footer>
</body>ARIA & Accessibility
ARIA (Accessible Rich Internet Applications) attributes extend HTML semantics for assistive technologies. The first rule of ARIA: don't use ARIA if native HTML semantics work.
<!-- Live region: announces dynamic content to screen readers -->
<div role="status" aria-live="polite" aria-atomic="true"></div>
<div role="alert" aria-live="assertive" ></div>
<!-- Modal dialog -->
<div role="dialog" aria-modal="true"
aria-labelledby="dlg-title" aria-describedby="dlg-desc">
<h2 id="dlg-title">Confirm Action</h2>
<p id="dlg-desc">Are you sure?</p>
</div>
<!-- Tab widget -->
<div role="tablist" aria-label="Settings">
<button role="tab" aria-selected="true" aria-controls="panel-1">General</button>
<button role="tab" aria-selected="false" aria-controls="panel-2">Privacy</button>
</div>
<div role="tabpanel" id="panel-1" aria-labelledby="tab-1">…</div>
<div role="tabpanel" id="panel-2" aria-labelledby="tab-2" hidden>…</div>
<!-- Toggle button -->
<button aria-pressed="false">Dark Mode</button>
<!-- Expandable menu -->
<button aria-expanded="false" aria-controls="submenu">Products ▾</button>
<ul id="submenu" hidden>…</ul>
<!-- Combobox / autocomplete -->
<input role="combobox" aria-expanded="true"
aria-autocomplete="list" aria-controls="results-list" />
<!-- Visually hidden but accessible text -->
<span class="sr-only">Loading, please wait…</span>
<!-- CSS: .sr-only { position:absolute; width:1px; height:1px; overflow:hidden; clip:rect(0,0,0,0); } -->
<!-- aria-hidden: hide from AT but keep visible -->
<span aria-hidden="true">★★★☆☆</span>
<span class="sr-only">3 out of 5 stars</span>SEO & Meta Tags
This is a
Modern HTML Elements
<!-- Native Dialog (modal) -->
<dialog id="my-dialog">
<h2>Dialog Title</h2>
<p>Dialog content with backdrop support.</p>
<button onclick="this.closest('dialog').close()">Close</button>
</dialog>
<button onclick="document.getElementById('my-dialog').showModal()">Open Dialog</button>
<!-- Details / Summary — native accordion -->
<details open>
<summary>Click to toggle</summary>
<p>Hidden content revealed on click. No JS needed!</p>
</details>
<!-- Popover API (2024) — No JS needed for basic use -->
<button popovertarget="my-popover">Show Popover</button>
<div id="my-popover" popover>
<p>Popover content. Click outside to dismiss.</p>
</div>
<!-- Output — result of a calculation -->
<form oninput="result.value = parseInt(a.value) + parseInt(b.value)">
<input type="number" id="a" value="0"/> +
<input type="number" id="b" value="0"/> =
<output name="result" for="a b">0</output>
</form>
<!-- Progress and Meter -->
<progress value="70" max="100">70%</progress>
<meter value="0.6" min="0" max="1"
low="0.25" high="0.75" optimum="0.8">
60% disk used
</meter>Web Components
Web Components are a suite of standards for building reusable, encapsulated custom HTML elements.
Performance & Loading
Script downloads in parallel, executes after HTML is fully parsed. Order preserved. Use for most scripts.
defer
Script downloads in parallel, executes after HTML is fully parsed. Order preserved. Use for most scripts.
async
Downloads in parallel, executes immediately when ready. No order guarantee. Best for independent analytics.
type="module"
ES Modules. Deferred by default. Enables import/export syntax. Always strict mode. CORS required.
loading="lazy"
Browser-native lazy loading for images and iframes. Loads only when near the viewport.
<head>
<!-- Preconnect: early TCP+TLS handshake to origin -->
<link rel="preconnect" href="https://fonts.googleapis.com"/>
<link rel="preconnect" href="https://cdn.example.com" crossorigin/>
<!-- DNS Prefetch: cheaper than preconnect -->
<link rel="dns-prefetch" href="https://analytics.example.com"/>
<!-- Preload: fetch critical resource before parser finds it -->
<link rel="preload" href="hero.jpg" as="image"/>
<link rel="preload" href="font.woff2" as="font" crossorigin/>
<link rel="preload" href="critical.css"as="style"/>
<!-- Prefetch: fetch next-page resource in idle time -->
<link rel="prefetch" href="/next-page.html"/>
<!-- Speculation Rules API (Chrome 2023) -->
<script type="speculationrules">
{ "prerender": [{ "source": "list", "urls": ["/dashboard"] }] }
</script>
</head>
<body>
<!-- LCP image: eager load, priority fetch -->
<img src="hero.jpg" alt="Hero"
loading="eager"
fetchpriority="high"
decoding="sync"
/>
<!-- Below-fold images: lazy load -->
<img src="below-fold.jpg" alt="…"
loading="lazy"
decoding="async"
/>
<!-- Script loading strategies -->
<script src="app.js" defer></script> <!-- parse HTML, execute after -->
<script src="widget.js" async></script> <!-- fetch in parallel, execute ASAP -->
<script src="module.js" type="module"></script><!-- ESM: deferred by default -->
<script src="worker-hint.js" blocking="render"></script>
</body>HTML APIs
Write semantic HTML before adding CSS or JS. Your HTML alone should make sense.
<!-- Draggable source -->
<div draggable="true"
ondragstart="e.dataTransfer.setData('text/plain', 'Hello')">
Drag me!
</div>
<!-- Drop target -->
<div ondragover="event.preventDefault()"
ondrop="e.target.textContent = e.dataTransfer.getData('text/plain')">
Drop here
</div><!-- Any element can become editable -->
<article
contenteditable="true"
spellcheck="true"
role="textbox"
aria-multiline="true"
aria-label="Edit article content"
>
Click to edit this paragraph…
</article>
<!-- execCommand (legacy but still used) -->
<button onclick="document.execCommand('bold')">Bold</button>Best Practices & Checklist
Structure First
Write semantic HTML before adding CSS or JS. Your HTML alone should make sense.
sandboxStructure First
Write semantic HTML before adding CSS or JS. Your HTML alone should make sense.
Accessibility
Test with a keyboard only. Run axe or WAVE. Add alt text, labels, skip links, and ARIA where needed.
Mobile First
Include viewport meta tag. Use responsive images. Test on real devices.
Performance
Lazy-load below-fold images. Defer non-critical JS. Preload LCP image. Use modern formats (WebP, AVIF).
Security
Sanitize user input before inserting to DOM. Use sandbox on iframes. Set rel="noopener" on external links.
sandboxSEO
One H1 per page. Descriptive title + meta desc. Canonical URLs. JSON-LD structured data. Valid HTML.
✅ DOCTYPE declaration present
✅ <html lang="…"> set correctly
✅ <meta charset="UTF-8"> is first in <head>
✅ Viewport meta tag present
✅ <title> is descriptive and unique per page
✅ Meta description is present (150-160 chars)
✅ One <h1> per page, headings in order
✅ All images have alt attributes
✅ All inputs have associated <label> elements
✅ External links have rel="noopener noreferrer"
✅ iframes have title attributes
✅ Color contrast ratio ≥ 4.5:1 (WCAG AA)
✅ Interactive elements are keyboard-reachable
✅ Skip link provided for keyboard users
✅ Form errors associated with inputs via aria-describedby
✅ Canonical URL set in <head>
✅ Open Graph tags present
✅ Favicon provided (at minimum 32×32 and 180×180)
✅ Scripts use defer or are at end of <body>
✅ No deprecated elements (center, font, frame, etc.)
✅ HTML validates at validator.w3.orgProve it on the ExamFlow Arena
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