/semantic-html-and-seo

name: semantic-html-and-seo
description: Semantic HTML5, SEO fundamentals, alt texts, progressive enhancement, SPA considerations, device capability detection, and user context awareness. Good HTML is the foundation of accessibility, SEO, and resilient UI. Use when building any web UI, reviewing markup quality, or optimising for search and accessibility.
metadata:
  priority: 8
  pathPatterns:
    - "**/*.html"
    - "**/*.tsx"
    - "**/*.jsx"
    - "**/*.vue"
    - "**/*.svelte"
    - "app/**"
    - "pages/**"
    - "src/**"
  promptSignals:
    phrases:
      - "seo"
      - "semantic html"
      - "alt text"
      - "meta tags"
      - "progressive enhancement"
      - "spa"
      - "open graph"
      - "structured data"
      - "html5"
      - "localstorage"
      - "sessionstorage"
      - "personalization"
      - "remember user preference"
retrieval:
  aliases:
    - seo
    - semantic html
    - alt text
    - progressive enhancement
    - html5 best practices
    - meta tags
    - open graph
    - structured data
    - spa seo
    - client-side storage
    - localStorage personalization
  intents:
    - improve seo
    - add alt texts
    - write semantic html
    - optimise for search engines
    - add open graph tags
    - make spa crawlable
    - persist user preferences client-side
    - personalise based on stored state
  examples:
    - add proper meta tags to this page
    - write semantic html for this component
    - how do I make this SPA SEO friendly
    - add alt texts to all images
    - remember the user's last view with localStorage
    - personalise this returning-user experience

Semantic HTML and SEO

Good HTML is not just markup — it is the contract between your content, search engines, assistive technologies, and the browser. Semantic HTML, correct metadata, and progressive enhancement make UI resilient, findable, and accessible by default.

---

Semantic HTML5

Use the element that describes the content's meaning, not just its appearance.

Document structure

<header>       <!-- site header, logo, primary nav -->
<nav>          <!-- navigation links -->
<main>         <!-- primary page content, one per page -->
<article>      <!-- self-contained content: blog post, product card, news item -->
<section>      <!-- thematic grouping with a heading -->
<aside>        <!-- tangentially related content: sidebar, callout -->
<footer>       <!-- site footer, secondary links, copyright -->

Headings

One `<h1>` per page — the primary topic. Headings form an outline: do not skip levels (`h1` → `h3` without `h2`).

<h1>Product name</h1>
  <h2>Features</h2>
    <h3>Feature detail</h3>
  <h2>Pricing</h2>

Interactive elements

<button>   <!-- clickable action, submits or triggers JS -->
<a href>   <!-- navigation to a URL -->
<input>    <!-- user data entry -->
<select>   <!-- option selection -->
<details> / <summary>  <!-- native disclosure/accordion -->

Never use `<div>` or `<span>` as interactive elements without full ARIA annotation — and even then, prefer the native element.

---

Images and Alt Text

Every `<img>` needs an `alt` attribute. What goes in it depends on context.

| Image type | Alt text | |---|---| | Informative (product photo, chart) | Describe content: `alt="Red leather sofa, three-seater"` | | Functional (icon button, logo link) | Describe function: `alt="Go to homepage"` | | Decorative | Empty: `alt=""` — screen readers skip it | | Complex (chart, diagram) | Short alt + longer description nearby or in `<figcaption>` |

<!-- Informative -->
<img src="sofa.jpg" alt="Red leather sofa, three-seater">

<!-- Decorative -->
<img src="divider.svg" alt="">

<!-- With caption -->
<figure>
  <img src="chart.png" alt="Bar chart showing revenue growth Q1–Q4 2025">
  <figcaption>Revenue grew 42% year-on-year in Q4 2025.</figcaption>
</figure>

---

SEO Fundamentals

Title and description

<title>Product Name — Short descriptor | Brand</title>
<meta name="description" content="One or two sentences. What this page is, who it is for, what they will find.">

• Title: 50–60 characters. Most important keyword first.
• Description: 120–160 characters. Shown in search results — write for the human, not the algorithm.

Canonical URL

<link rel="canonical" href="https://example.com/the-definitive-url">

Prevents duplicate content penalties when the same page is accessible via multiple URLs.

Open Graph (social sharing)

<meta property="og:title" content="Page title">
<meta property="og:description" content="Page description">
<meta property="og:image" content="https://example.com/og-image.jpg">
<meta property="og:url" content="https://example.com/page">
<meta property="og:type" content="website">

<!-- Twitter/X -->
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:title" content="Page title">
<meta name="twitter:image" content="https://example.com/og-image.jpg">

OG image: 1200×630px. Appears when the URL is shared on Slack, LinkedIn, Twitter, iMessage.

Structured Data (JSON-LD)

Machine-readable content enables rich search results.

<script type="application/ld+json">
{
  "@context": "https://schema.org",
  "@type": "Product",
  "name": "Product Name",
  "description": "Product description",
  "image": "https://example.com/product.jpg",
  "offers": {
    "@type": "Offer",
    "price": "49.00",
    "priceCurrency": "EUR"
  }
}
</script>

Common types: `Product`, `Article`, `BreadcrumbList`, `FAQPage`, `Organization`, `SiteLinksSearchBox`.

---

Progressive Enhancement

Build in layers. The core content and function must work without JavaScript. Enhance with CSS. Enhance further with JS.

Layer 1: HTML — content is readable, links work, forms submit
Layer 2: CSS  — layout, typography, visual design
Layer 3: JS   — interactivity, animations, dynamic content

**In practice:**

• Forms must submit via