WebPerf: Media Performance

JavaScript snippets for measuring web performance in Chrome DevTools. Execute with mcp__chrome-devtools__evaluate_script, capture output with mcp__chrome-devtools__get_console_message.

Scripts

• scripts/Image-Element-Audit.js — Image Element Audit
• scripts/SVG-Embedded-Bitmap-Analysis.js — SVG Embedded Bitmap Analysis
• scripts/Video-Element-Audit.js — Video Element Audit

Descriptions and thresholds: references/snippets.md

Common Workflows

Complete Media Audit

When the user asks for media optimization or "audit images and videos":

1. Image-Element-Audit.js - Analyze all images (format, lazy loading, sizing, fetchpriority)
2. Video-Element-Audit.js - Analyze all videos (poster, preload, formats, autoplay)
3. SVG-Embedded-Bitmap-Analysis.js - Detect inefficient bitmap images embedded in SVGs

Image Optimization Workflow

When the user asks "optimize images" or "check image performance":

1. Image-Element-Audit.js - Full image audit
2. Cross-reference with webperf-loading skill:
   • Find-Above-The-Fold-Lazy-Loaded-Images.js (incorrectly lazy-loaded images)
   • Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js (missing lazy loading)
   • Find-Images-With-Lazy-and-Fetchpriority.js (contradictory attributes)
   • Priority-Hints-Audit.js (LCP image should have fetchpriority="high")

Video Performance Audit

When the user asks "optimize videos" or "check video performance":

1. Video-Element-Audit.js - Full video audit
2. Cross-reference with webperf-core-web-vitals skill:
   • LCP-Video-Candidate.js (check if video/poster is LCP)
3. Cross-reference with webperf-loading skill:
   • Priority-Hints-Audit.js (video poster priority)
   • Resource-Hints-Validation.js (video preload)

LCP Image Investigation

When LCP is an image and needs optimization:

1. Cross-reference with webperf-core-web-vitals skill:
   • LCP.js (measure LCP)
   • LCP-Image-Entropy.js (analyze image complexity)
2. Image-Element-Audit.js - Check format, dimensions, lazy loading
3. Cross-reference with webperf-loading skill:
   • Find-Above-The-Fold-Lazy-Loaded-Images.js (should NOT be lazy)
   • Priority-Hints-Audit.js (should have fetchpriority="high")
   • Resource-Hints-Validation.js (consider preload)

Layout Shift from Images

When CLS is caused by images without dimensions:

1. Image-Element-Audit.js - Check for missing width/height attributes
2. Cross-reference with webperf-core-web-vitals skill:
   • CLS.js (measure total CLS)
3. Cross-reference with webperf-interaction skill:
   • Layout-Shift-Loading-and-Interaction.js (when shifts occur)

SVG Optimization Audit

When the user asks about SVG performance or file sizes are large:

1. SVG-Embedded-Bitmap-Analysis.js - Detect raster images embedded in vector SVGs
2. Recommend SVGO optimization for SVGs without embedded bitmaps
3. Recommend extracting bitmaps to separate image files with proper formats

Decision Tree

Use this decision tree to automatically run follow-up snippets based on results:

After Image-Element-Audit.js

• If images missing width/height attributes → Layout shift risk, run:

  1. webperf-core-web-vitals:CLS.js (measure CLS impact)
  2. webperf-interaction:Layout-Shift-Loading-and-Interaction.js (timing of shifts)
  3. Recommend adding explicit dimensions to all images

• If images using wrong format (JPEG for graphics, PNG for photos) → Recommend:

  • Modern formats: WebP, AVIF
  • Appropriate format for content type
  • Format-specific compression settings

• If images much larger than display size → Recommend:

  • Responsive images with srcset
  • Appropriate image CDN sizing
  • srcset with multiple sizes for different viewports

• If above-the-fold images are lazy-loaded → Run:

  1. webperf-loading:Find-Above-The-Fold-Lazy-Loaded-Images.js (confirm)
  2. webperf-core-web-vitals:LCP.js (measure LCP impact)
  3. Recommend removing loading="lazy" from above-fold images

• If LCP image lacks fetchpriority="high" → Run:

  1. webperf-core-web-vitals:LCP.js (measure current LCP)
  2. webperf-loading:Priority-Hints-Audit.js (full priority audit)
  3. Recommend adding fetchpriority="high" to LCP image

• If below-the-fold images are NOT lazy-loaded → Run:

  1. webperf-loading:Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js (confirm)
  2. Recommend adding loading="lazy" to offscreen images

• If images have both loading="lazy" AND fetchpriority="high" → Run:

  1. webperf-loading:Find-Images-With-Lazy-and-Fetchpriority.js (confirm contradiction)
  2. Recommend removing one of the conflicting attributes

• If images missing alt text → Accessibility issue, recommend adding descriptive alt text

After Video-Element-Audit.js

• If video is LCP candidate → Run:

  1. webperf-core-web-vitals:LCP-Video-Candidate.js (confirm)
  2. webperf-core-web-vitals:LCP.js (measure LCP)
  3. webperf-core-web-vitals:LCP-Sub-Parts.js (break down timing)
  4. Optimize video poster image or consider image alternative

• If video missing poster → Recommend:

  • Adding poster image for better perceived performance
  • Using first frame or custom thumbnail
  • Optimizing poster as you would an image

• If video uses preload="auto" → Bandwidth concern, evaluate:

  • Is video above-the-fold? Keep preload="auto"
  • Is video below-the-fold? Change to preload="metadata" or "none"
  • Is autoplay intended? Verify preload matches intent

• If autoplay video without muted → Browser will block, recommend:

  • Adding muted attribute
  • Or removing autoplay

• If video missing multiple formats → Recommend:

  • WebM for Chrome/Firefox
  • MP4 as fallback for Safari
  • Order sources by efficiency (WebM first)

• If large video files (>5MB) → Recommend:

  • Compression/transcoding
  • Adaptive bitrate streaming (HLS, DASH)
  • Loading strategy optimization

After SVG-Embedded-Bitmap-Analysis.js

• If bitmap images found in SVGs → Recommend:

  1. Extract bitmaps to separate files
  2. Use WebP/AVIF format for extracted images
  3. Reference images from SVG with <image> element
  4. Or convert to pure vector if possible

• If large embedded bitmaps (>100KB) → Critical inefficiency:

  • SVG parsing overhead + large bitmap = worst of both worlds
  • Urgently recommend extraction

• If multiple small bitmaps in SVG → Consider:

  • CSS sprites for small icons
  • SVG symbols for reusable graphics
  • Extracting to individual optimized images

Cross-Skill Triggers

These triggers recommend using snippets from other skills:

From Media to Core Web Vitals Skill

• If LCP image detected → Use webperf-core-web-vitals skill:

  • LCP.js (measure LCP)
  • LCP-Sub-Parts.js (break down timing phases)
  • LCP-Image-Entropy.js (analyze image complexity)

• If video is LCP candidate → Use webperf-core-web-vitals skill:

  • LCP-Video-Candidate.js (confirm and analyze)
  • LCP.js (measure impact)

• If images causing layout shifts → Use webperf-core-web-vitals skill:

  • CLS.js (measure cumulative shift)

From Media to Loading Skill

• If lazy loading issues detected → Use webperf-loading skill:

  • Find-Above-The-Fold-Lazy-Loaded-Images.js (incorrectly lazy)
  • Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js (missing lazy)
  • Find-Images-With-Lazy-and-Fetchpriority.js (contradictory attributes)

• If LCP image needs priority optimization → Use webperf-loading skill:

  • Priority-Hints-Audit.js (fetchpriority analysis)
  • Resource-Hints-Validation.js (preload validation)

• If images competing with critical resources → Use webperf-loading skill:

  • Find-render-blocking-resources.js (resource priority conflicts)
  • TTFB-Resources.js (identify slow image CDN)

From Media to Interaction Skill

• If images causing layout shifts during interaction → Use webperf-interaction skill:
  • Layout-Shift-Loading-and-Interaction.js (shift timing analysis)

Performance Budget Thresholds

Use these thresholds to trigger recommendations:

Image File Sizes:

• Warning: Individual image > 500KB → Check format and compression
• Critical: Individual image > 1MB → Urgent optimization needed
• Total images: > 5MB on initial load → Implement lazy loading

Image Formats:

• JPEG for graphics/icons → Recommend PNG or SVG
• PNG for photos → Recommend JPEG, WebP, or AVIF
• GIF for animations → Recommend video (MP4/WebM) or animated WebP
• No modern formats (WebP/AVIF) → Recommend upgrading

Image Dimensions:

• Intrinsic size > 2x display size → Recommend responsive images
• Intrinsic size < display size → Upscaling = blurry, provide larger source

Video File Sizes:

• Warning: Video > 10MB → Consider compression or streaming
• Critical: Video > 50MB → Urgent optimization or streaming needed

Lazy Loading:

• Above-fold images lazy-loaded → Critical LCP impact, fix immediately
• Below-fold images NOT lazy-loaded → Wasted bandwidth, implement lazy loading
• >10 images eager-loaded → Excessive, implement lazy loading

Priority Hints:

• LCP image without fetchpriority="high" → Add for 10-30% LCP improvement
• Non-LCP images with fetchpriority="high" → Remove, wasting browser hints
• Lazy + fetchpriority="high" conflict → Fix contradiction

Common Issues and Resolutions

Issue: LCP is slow and LCP element is an image

1. Run Image-Element-Audit.js
2. Run webperf-core-web-vitals:LCP-Image-Entropy.js
3. Check: format, lazy loading, fetchpriority, preload
4. Fix in order: remove lazy, add fetchpriority="high", optimize format, add preload

Issue: CLS from images

1. Run Image-Element-Audit.js
2. Check for missing width/height
3. Add explicit dimensions or aspect-ratio CSS
4. Verify with webperf-core-web-vitals:CLS.js

Issue: Page loads too many images

1. Run Image-Element-Audit.js
2. Run webperf-loading:Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js
3. Implement lazy loading on below-fold images
4. Consider pagination or infinite scroll

Issue: Images are the wrong format

1. Run Image-Element-Audit.js
2. Check format vs content type
3. Recommend WebP with JPEG/PNG fallback
4. Consider AVIF for even better compression

Issue: Video is LCP

1. Run Video-Element-Audit.js
2. Run webperf-core-web-vitals:LCP-Video-Candidate.js
3. Optimize poster image or consider static image alternative
4. Add fetchpriority="high" to poster if keeping video

Issue: SVG files are huge

1. Run SVG-Embedded-Bitmap-Analysis.js
2. Extract embedded bitmaps
3. Run SVGO on pure SVG
4. Re-measure file sizes

References

• references/snippets.md — Descriptions and thresholds for each script
• references/schema.md — Return value schema for interpreting script output

Execute via mcp__chrome-devtools__evaluate_script → read with mcp__chrome-devtools__get_console_message.