diff --git a/.auto-claude/specs/001-conceptual-explanations/build-progress.txt b/.auto-claude/specs/001-conceptual-explanations/build-progress.txt
index ce927e3..2b21e7c 100644
--- a/.auto-claude/specs/001-conceptual-explanations/build-progress.txt
+++ b/.auto-claude/specs/001-conceptual-explanations/build-progress.txt
@@ -174,7 +174,7 @@ Added conceptual explanations to all 6 CSS Grid lessons.
 
 Implementation details:
 - Added 'concept' objects to all Grid lessons explaining the 2D grid system, tracks, and cell placement
-- Lesson 1 (Grid Container Basics): 
+- Lesson 1 (Grid Container Basics):
   * Explanation of 2D layout system, tracks (rows/columns), 1fr units, and gap property
   * Diagram showing grid container with 3 equal columns and 2 rows
   * Container vs Item: display: grid, grid-template-columns, and gap are container properties
@@ -377,3 +377,77 @@ Implementation details:
 Committed: a7f0761
 Status: ✓ Completed
 
+=== 2026-01-11 - Subtask 4.6 Completed ===
+Added conceptual explanations to responsive design module (08-responsive.json).
+
+Implementation details:
+- Added 'concept' objects to all 4 lessons explaining media queries, breakpoints, and mobile-first design principles
+- Lesson 1 (Media Queries):
+  * Explanation of how media queries are conditional CSS rules evaluated continuously by the browser
+  * Shows how @media (max-width: 600px) checks viewport width in real-time
+  * Diagram illustrating browser evaluation process and breakpoint behavior
+  * Visual examples showing cascade with media queries (source order matters)
+  * Common media features reference: width, height, orientation, prefers-color-scheme, hover
+  * Explains how media query styles override base styles when conditions match
+- Lesson 2 (Fluid Type):
+  * Explanation of viewport units (vw, vh, vmin, vmax) scaling proportionally with window size
+  * Shows calculation: 5vw on 1000px screen = 50px (5% of viewport width)
+  * Diagram showing how font size changes across mobile (375px), tablet (768px), and desktop (1440px)
+  * Identifies problem with unbounded scaling (too small on mobile, too large on wide screens)
+  * Solution: clamp(16px, 5vw, 48px) to set minimum and maximum bounds
+  * Best practices: use for hero headings, avoid for body text
+- Lesson 3 (Responsive Grid):
+  * Explanation of auto-fit with minmax() creating intrinsically responsive grids without media queries
+  * Pattern: repeat(auto-fit, minmax(200px, 1fr)) means "as many columns as fit, at least 200px each"
+  * Diagram showing step-by-step calculation and responsive reflow behavior (4→3→2→1 columns)
+  * Visual comparison of auto-fit vs auto-fill (collapsing vs preserving empty tracks)
+  * Shows natural breakpoints calculated automatically by browser
+  * Container vs Item: display: grid, grid-template-columns, gap are container properties
+- Lesson 4 (Mobile-First):
+  * Explanation of mobile-first approach: base CSS for mobile, min-width queries for enhancements
+  * Three key advantages: less CSS for mobile users, forces content prioritization, cascade works in favor
+  * Diagram comparing mobile-first vs desktop-first design patterns and CSS flow
+  * Performance benefits shown: mobile-first parses 2KB vs desktop-first 4KB on mobile devices
+  * Common breakpoints: 768px (tablet), 1024px (desktop), 1280px (large desktop)
+  * Visual comparison of content prioritization: mobile shows core content, desktop adds extras
+  * Why min-width is better: progressive enhancement, aligns with cascade, easier to maintain
+- All explanations are beginner-friendly (2-4 sentences)
+- Detailed ASCII diagrams illustrate media query evaluation, viewport calculations, grid reflow, and design patterns
+- Focus on WHY these responsive techniques work and WHEN to use each approach
+- Covers fundamental responsive concepts: conditional CSS, fluid units, intrinsic layouts, progressive enhancement
+- Phase 4 (Content - Visual & Layout Modules) is now complete - all 6 subtasks finished
+
+Committed: 79b858e
+Status: ✓ Completed
+
+=== 2026-01-11 - Subtask 5.2 Completed ===
+Added conceptual explanations to HTML form modules (21-html-forms-basic.json and 22-html-forms-validation.json).
+
+Implementation details:
+- Added concept objects to all 6 lessons across both form modules
+- Covers native form validation, input types, and accessibility patterns
+- All explanations are beginner-friendly (2-4 sentences)
+- ASCII diagrams illustrate form accessibility, validation flows, and constraint behaviors
+
+Committed: 85f2aa4
+Status: ✓ Completed
+
+=== 2026-01-11 - Subtask 5.4 Completed ===
+Added conceptual explanations to Tailwind basics module (10-tailwind-basics.json).
+
+Implementation details:
+- Added 'concept' objects to all 5 Tailwind lessons explaining utility-first approach and how it differs from traditional CSS
+- Lesson 1 (Backgrounds): Pre-built utilities vs custom CSS, color scale system (50-950), no naming or context-switching
+- Lesson 2 (Utility-First Philosophy): Workflow inversion, problems solved (naming, specificity, dead CSS), tradeoffs
+- Lesson 3 (Text Utilities): Naming patterns (property-value), shade system for accessibility, predictable conventions
+- Lesson 4 (Spacing): Base-4 scale (0.25rem increments), directional shorthands (px/py), mx-auto centering
+- Lesson 5 (Breakpoints): Mobile-first responsive system, breakpoint prefixes (sm/md/lg/xl/2xl), inline media queries
+- All explanations are beginner-friendly (2-4 sentences)
+- Detailed ASCII diagrams illustrate utility-first concepts and comparisons with traditional CSS
+- Clear explanations of how Tailwind differs from traditional CSS
+- Focus on WHY Tailwind works differently and WHAT problems it solves
+- Phase 5 (Content - HTML & Tailwind Modules) is now complete - all 4 subtasks finished
+
+Committed: dfd9062
+Status: ✓ Completed
+