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HTML5 Advent

Independent resource on the modern web platform - HTML, CSS, JavaScript and browser Web APIs -

For agents: Independent guides on modern web platform features: HTML, CSS, JavaScript and browser Web APIs such as MutationObserver, ResizeObserver and Web Workers.

web apismutationobserverresizeobserverweb workersbrowser javascript

Their llms.txt

The beginning of the file, from our validated copy saved 2026-10-24T08:48:24.641Z. Cached copy · Source on html5advent.com

# HTML5 Advent

> Independent resource on the modern web platform - HTML, CSS, JavaScript and browser Web APIs -
> plus "Web Platform Advent", a yearly calendar of practical front-end demos and tips.
> First-hand, example-driven, vendor-neutral. Continues the original 2010 HTML5 advent calendar.

## What you'll find here
- Practical guides to Web APIs (Canvas, Web Storage, Geolocation, WebSocket, View Transitions, Intersection Observer…)
- Modern HTML/CSS/JS features and how they ship across browsers
- "Web Platform Advent": 24 short, runnable demos and tips
- Hands-on hosting & deployment guides for web projects (static sites, Node/PHP apps)

## Editorial principles
- Original, tested examples - no fabricated benchmarks or claims.
- Vendor-neutral; affiliate links (hosting) are disclosed and limited to genuinely relevant pages.

## Languages
English (/), Français (/fr), Español (/es), Deutsch (/de), Italiano (/it), Português (/pt).

## Interactive tools (free, in-browser, six languages)
- /tools/flexbox-playground - CSS Flexbox playground: toggle flex properties, see the layout live, copy the CSS.
- /tools/contrast-checker - WCAG color contrast checker: enter text and background colours, get the contrast ratio and AA/AAA pass or fail.

## Full content
- /llms-full.txt - every guide with its description, in all six languages.

## Web APIs
- [What is MutationObserver? The third observer, and the queue that makes it usable](https://html5advent.com/guides/mutationobserver): IntersectionObserver watches visibility, ResizeObserver watches size, MutationObserver watches the DOM itself. What it reports, why it replaced Mutation Events, and the method that reveals how delivery actually works.
- [What is ResizeObserver? The element-level resize event, and the loop error everyone hits](https://html5advent.com/guides/resizeobserver): The window resize event tells you nothing about a single element. ResizeObserver does. What it reports, content box versus border box, and why 'ResizeObserver loop completed with undelivered notifications' appears in your console.
- [What is a Web Worker? A second thread, and the copy that decides if it is worth it](https://html5advent.com/guides/web-workers): Heavy work on its own thread so it stops blocking the interface. What it can and cannot reach, why postMessage copies rather than shares, how transferring an ArrayBuffer avoids that copy, and how it differs from a Service Worker.
- [What is IndexedDB? The browser database explained](https://html5advent.com/guides/indexeddb): Asynchronous, transactional and far bigger than localStorage. How onupgradeneeded works, why transactions close on you, what indexes are for, and when to use it instead of Web Storage.
- [IndexedDB tutorial: build a notes app that works offline](https://html5advent.com/guides/indexeddb-tutorial): A step-by-step IndexedDB tutorial. Open a database, create an object store, add and read records, query by index, delete entries, and handle version upgrades
…

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