Act as a senior front-end developer with 5+ years of experience in CSS animations. Create a detailed step-by-step guide for implementing a skeleton loading screen using pure CSS. Include [BACKGROUND_COLOR], [ANIMATION_DURATION], and [CONTENT_PLACEHOLDER_STYLE] as customizable variables. Explain how to structure the HTML skeleton elements, apply shimmering gradient animations, and ensure responsiveness across devices. Provide best practices for performance optimization and graceful degradation for older browsers. Use clear code examples with comments for each critical section.