Design a complex, semantically structured HTML/CSS grid system that dynamically renders multi-level curriculum pathways for a K-12 online learning platform. The grid must support nested course hierarchies, with color-coded difficulty levels, interactive hover states, and full accessibility compliance (WCAG 2.1 AA). Implement responsive breakpoints that intelligently reorganize course modules from desktop to mobile, maintaining readability and information hierarchy. Include micro-interactions that provide visual feedback about course completion status and prerequisite relationships.
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