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Custom Elements
Define custom element Autonomous custom element Custom element constructor connectedCallback disconnectedCallback attributeChangedCallback observedAttributes Custom element upgrade
Shadow DOM
attachShadow Open shadow root Closed shadow root Shadow tree styles Style encapsulation CSS custom properties across shadow ::part exportparts
Templates & Slots
template element DocumentFragment cloning slot element Named slots Default slot slotchange assignedElements Fallback slot content
Attributes & Properties
Reflect property to attribute Boolean attributes Property-only complex values Attribute parsing Custom events Event composed option Event bubbling across shadow Form-associated concept
Styling & APIs
adoptedStyleSheets Constructable stylesheet ElementInternals ARIA via internals Custom states concept Declarative shadow DOM Scoped registry concept Lifecycle-safe listeners
Production Patterns
Lazy definition define idempotently Versioning tag names SSR hydration strategy Testing shadow DOM Accessibility naming Performance template reuse Progressive enhancement
Web Components / Production Patterns

Progressive enhancement

Learn Progressive enhancement in Web Components with a dedicated explanation, a concept-specific example, a production caution, and a mastery check.

12 min read • Advanced
web-components-048.js
javascript 
<form action="/subscribe" method="post"><input name="email" type="email" required><button>Subscribe</button></form>
<script type="module">if("customElements" in window){import("./subscription-form.js")}</script>
What to watch
Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

Progressive enhancement Mastery Check →
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Performance template reuse