docs(perf): update image best practices and TTFB thresholds based on feedback

- Add width/height to <source> elements for correct aspect-ratio in art direction.
- Use realistic 'sizes' attributes in hero image examples.
- Remove 'decoding=sync' from hero images to avoid blocking the main thread.
- Align TTFB 'good' threshold with web.dev standards (800ms).

references/performance-checklist.md

@@ -20,7 +20,7 @@ Quick reference checklist for web application performance. Use alongside the `pe
 
 ## TTFB Diagnosis
 
-When TTFB is slow (> 600ms), check each component in DevTools Network waterfall:
+When TTFB is slow (> 800ms), check each component in DevTools Network waterfall:
 
 - [ ] **DNS resolution** slow → add `<link rel="dns-prefetch">` or `<link rel="preconnect">` for known origins
 - [ ] **TCP/TLS handshake** slow → enable HTTP/2, consider edge deployment, verify keep-alive
@@ -31,9 +31,9 @@ When TTFB is slow (> 600ms), check each component in DevTools Network waterfall:
 ### Images
 - [ ] Images use modern formats (WebP, AVIF)
 - [ ] Images are responsively sized (`srcset` and `sizes`)
-- [ ] Images have explicit `width` and `height` attributes and/or CSS `aspect-ratio` (prevents CLS)
+- [ ] Images and `<source>` elements have explicit `width` and `height` (prevents CLS in art direction)
 - [ ] Below-the-fold images use `loading="lazy"` and `decoding="async"`
-- [ ] Hero/LCP images use `fetchpriority="high"`, `decoding="sync"`, and no lazy loading
+- [ ] Hero/LCP images use `fetchpriority="high"` and no lazy loading
 
 ### JavaScript
 - [ ] Bundle size under 200KB gzipped (initial load)

skills/performance-optimization/SKILL.md

@@ -162,35 +162,43 @@ const tasks = await db.tasks.findMany({
   - Resolution switching (srcset + sizes): right file size per screen density
 -->
 <picture>
-  <!-- Mobile: portrait crop -->
+  <!-- Mobile: portrait crop (8:10) -->
   <source
     media="(max-width: 767px)"
     srcset="/hero-mobile-400.avif 400w, /hero-mobile-800.avif 800w"
     sizes="100vw"
+    width="800"
+    height="1000"
     type="image/avif"
   />
   <source
     media="(max-width: 767px)"
     srcset="/hero-mobile-400.webp 400w, /hero-mobile-800.webp 800w"
     sizes="100vw"
+    width="800"
+    height="1000"
     type="image/webp"
   />
-  <!-- Desktop: landscape crop -->
+  <!-- Desktop: landscape crop (2:1) -->
   <source
     srcset="/hero-800.avif 800w, /hero-1200.avif 1200w, /hero-1600.avif 1600w"
-    sizes="100vw"
+    sizes="(max-width: 1200px) 100vw, 1200px"
+    width="1200"
+    height="600"
     type="image/avif"
   />
   <source
     srcset="/hero-800.webp 800w, /hero-1200.webp 1200w, /hero-1600.webp 1600w"
-    sizes="100vw"
+    sizes="(max-width: 1200px) 100vw, 1200px"
+    width="1200"
+    height="600"
     type="image/webp"
   />
   <img
     src="/hero-desktop.jpg"
-    style="aspect-ratio: 1200 / 600"
+    width="1200"
+    height="600"
     fetchpriority="high"
-    decoding="sync"
     alt="Hero image description"
   />
 </picture>