progressive-enhancementlisted

# Progressive Enhancement Accessibility Skill > **Canonical source**: `examples/PROGRESSIVE_ENHANCEMENT_BEST_PRACTICES.md` in `mgifford/ACCESSIBILITY.md` > This skill is derived from that file. When in doubt, the example is authoritative. Apply these rules when building any web feature or reviewing architecture decisions. --- ## Core Mandate Start with a solid foundation that works for every user, then layer enhancements. Every user — regardless of browser capability, network speed, assistive technology, or JavaScript availability — must be able to access core content and complete core tasks. Progressive enhancement is also a sustainability practice. Pages that work without JavaScript are dramatically lighter: fewer bytes transferred, less CPU spent on parsing and executing scripts, and less energy consumed per page view. See [SUSTAINABILITY.md](https://github.com/mgifford/SUSTAINABILITY.md) and the [Web Sustainability Guidelines](https://www.w3.org/TR/web-sustainability-guidelines/). --- ## Severity Scale (this skill) | Level | Meaning | | --- | --- | | **Critical** | Core content or task inaccessible without JS/CSS | | **Serious** | Core content accessible but significantly degraded without JS/CSS | | **Moderate** | Enhancement degrades gracefully but with friction | | **Minor** | Best-practice gap; marginal impact | --- ## The Three Layers ### Layer 1 — Semantic HTML (always required) **Failure here is Critical.** * All core content readable in plain HTML — n