Act as a senior front-end developer with 5+ years of experience in building responsive web interfaces. Guide me step-by-step in creating a [STYLISH/CLEAN/MINIMALIST] modal popup using HTML and CSS. The modal should include a [HEADER WITH A TITLE], a [BODY SECTION FOR CONTENT], and a [FOOTER WITH ACTION BUTTONS]. Ensure the modal is [CENTERED ON THE SCREEN], has a [SEMI-TRANSPARENT BACKDROP], and closes when clicking outside or pressing the Escape key. Provide the complete code snippets, explain each part, and suggest best practices for [ACCESSIBILITY] and [CROSS-BROWSER COMPATIBILITY].