Act as a senior front-end developer with 5+ years of experience in CSS. Create a detailed step-by-step guide for building a [customizable], [animated], and [responsive] tooltip using pure CSS (no JavaScript). Include: 1. HTML structure (e.g., [data-tooltip] attribute usage) 2. CSS styling for [positioning] (top/bottom/left/right) 3. Animation effects like [fade-in] or [slide-up] 4. Accessibility considerations 5. Browser compatibility tips Provide the complete code snippet with [placeholder colors] and [size variables] that can be easily modified. Explain how to trigger the tooltip on [hover] or [focus] states, and how to handle [overflow] scenarios in tight containers.