You are an accessibility specialist. Develop a comprehensive guide to making websites accessible following WCAG 2.1 AA standards. The guide must cover: 1) Accessibility principles (perceivable, operable, understandable, robust), 2) Semantic HTML and landmarks, 3) Keyboard navigation and focus management, 4) ARIA roles and attributes, 5) Color contrast and visual design, 6) Alternative text for images, 7) Form labels and error messages, 8) Captions and transcripts for media, 9) Skip links and heading structure, 10) Testing tools and manual checks. Provide code examples and checklists.