webperf-core-web-vitals

# WebPerf: Core Web Vitals 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/CLS.js` — Cumulative layout shift (CLS) - `scripts/INP.js` — Interaction to next paint (INP) - `scripts/LCP-Image-Entropy.js` — LCP image entropy - `scripts/LCP-Subparts.js` — LCP subparts - `scripts/LCP-Trail.js` — LCP trail - `scripts/LCP-Video-Candidate.js` — LCP video candidate - `scripts/LCP.js` — Largest contentful paint (LCP) ## Common Workflows ### Complete Core Web Vitals Audit When the user asks for a comprehensive Core Web Vitals analysis or "audit CWV": 1. **LCP.js** - Measure Largest Contentful Paint 2. **CLS.js** - Measure Cumulative Layout Shift 3. **INP.js** - Measure Interaction to Next Paint 4. **LCP-Subparts.js** - Break down LCP timing phases 5. **LCP-Trail.js** - Track LCP candidate evolution ### LCP Deep Dive When LCP is slow or the user asks "debug LCP" or "why is LCP slow": 1. **LCP.js** - Establish baseline LCP value 2. **LCP-Subparts.js** - Break down into TTFB, resource load, render delay 3. **LCP-Trail.js** - Identify all LCP candidates and changes 4. **LCP-Image-Entropy.js** - Check if LCP image has visual complexity issues 5. **LCP-Video-Candidate.js** - Detect if LCP is a video (poster or video element) ### CLS Investigation When layout shifts are detected or the user asks "debug CLS" or "layout shift issues": 1. **CLS.js** - Measure overall CLS score 2. **Layout-Shift-Loading-and-Interaction.js** _(pending — available in webperf-interaction skill)_ 3. Cross-reference with **webperf-loading** skill: - Find-Above-The-Fold-Lazy-Loaded-Images.js (lazy images causing shifts) - Fonts-Preloaded-Loaded-and-used-above-the-fold.js (font swap causing shifts) ### INP Debugging When interactions feel slow or the user asks "debug INP" or "slow interactions": 1. **INP.js** - Measure overall INP value; call `getINP()` after interactions, `getINPDetails()` for full list 2. **Interactions.js** _(pending — available in webperf-interaction skill)_ 3. **Input-Latency-Breakdown.js** _(pending — available in webperf-interaction skill)_ 4. **Long-Animation-Frames.js** _(pending — available in webperf-interaction skill)_ 5. **Long-Animation-Frames-Script-Attribution.js** _(pending — available in webperf-interaction skill)_ ### Video as LCP Investigation When LCP is a video element (detected by LCP-Video-Candidate.js): 1. **LCP-Video-Candidate.js** - Identify video as LCP candidate 2. **Video-Element-Audit.js** (from Media skill) - Audit video loading strategy 3. **LCP-Subparts.js** - Analyze video loading phases 4. Cross-reference with **webperf-loading** skill: - Resource-Hints-Validation.js (check for video preload) - Priority-Hints-Audit.js (check for fetchpriority on video) ### Image as LCP Investigation When LCP is an image (most common case): 1. **LCP.js** - Measure LCP timing 2. **LCP-Subparts.js** - Break down timing phases 3. **LCP-Image-Entropy.js** - Analyze image complexity 4. Cross-reference with **webperf-media** skill: - Image-Element-Audit.js (check format, dimensions, lazy loading) 5. Cross-reference with **webperf-loading** skill: - Find-Above-The-Fold-Lazy-Loaded-Images.js (check if incorrectly lazy) - Priority-Hints-Audit.js (check for fetchpriority="high") - Resource-Hints-Validation.js (check for preload) ## Decision Tree Use this decision tree to automatically run follow-up snippets based on results: ### After LCP.js - **If LCP > 2.5s** → Run **LCP-Subparts.js** to diagnose which phase is slow - **If LCP > 4.0s (poor)** → Run full LCP deep dive workflow (5 snippets) - **If LCP candidate is an image** → Run **LCP-Image-Entropy.js** and **webperf-media:Image-Element-Audit.js** - **If LCP candidate is a video** → Run **LCP-Video-Candidate.js** and **webperf-media:Video-Element-Audit.js** - **Always run** → **LCP-Trail.js** to understand candidate evolution ### After LCP-Subparts.js - **If TTFB phase > 600ms** → Switch to **webperf-loading** skill and run TTFB-Sub-Parts.js - **If Resource Load Time > 1500ms** → Run: 1. **webperf-loading:Resource-Hints-Validation.js** (check for preload/preconnect) 2. **webperf-loading:Priority-Hints-Audit.js** (check fetchpriority) 3. **webperf-loading:Find-render-blocking-resources.js** (competing resources) - **If Render Delay > 200ms** → Run: 1. **webperf-loading:Find-render-blocking-resources.js** (blocking CSS/JS) 2. **webperf-loading:Script-Loading.js** (parser-blocking scripts) 3. **webperf-interaction:Long-Animation-Frames.js** (main thread blocking) ### After LCP-Trail.js - **If many LCP candidate changes (>3)** → This causes visual instability, investigate: 1. **webperf-loading:Find-Above-The-Fold-Lazy-Loaded-Images.js** (late-loading images) 2. **webperf-loading:Fonts-Preloaded-Loaded-and-used-above-the-fold.js** (font swaps) 3. **CLS.js** (layout shifts contributing to LCP changes) - **If final LCP candidate appears late** → Run **webperf-loading:Resource-Hints-Validation.js** - **If early candidate was replaced** → Understand why initial content was pushed down (likely CLS issue) ### After LCP-Image-Entropy.js - **If entropy is very high** → Image is visually complex, recommend: - Modern formats (WebP, AVIF) - Appropriate compression - Potentially a placeholder strategy - **If entropy is low** → Image may be over-optimized or placeholder-like - **If large file size detected** → Run **webperf-media:Image-Element-Audit.js** for format/sizing analysis ### After LCP-Video-Candidate.js - **If video is LCP** → Run: 1. **webperf-media:Video-Element-Audit.js** (check poster, preload, formats) 2. **webperf-loading:Priority-Hints-Audit.js** (check fetchpriority on poster) 3. **LCP-Subparts.js** (analyze video loading phases) - **If poster image is LCP** → Treat as image LCP (run image workflows) ### After CLS.js - **If CLS > 0.1** → Run **webperf-interaction:Layout-Shift-Loading-and-Interaction.js** _(pending — available in webperf-interaction skill)_ - **If CLS > 0.25 (poor)** → Run comprehensive shift investigation: 1. **webperf-loading:Find-Above-The-Fold-Lazy-Loaded-Images.js** (images without dimensions) 2. **webperf-loading:Fonts-Preloaded-Loaded-and-used-above-the-fold.js** (font loading strategy) 3. **webperf-loading:Critical-CSS-Detection.js** (late-loading styles) 4. **webperf-media:Image-Element-Audit.js** (missing width/height) - **If CLS = 0** → Confirm with multiple page loads (might be timing-dependent) ### After INP.js - **If INP > 200ms** → Run **webperf-interaction:Interactions.js** to identify slow interactions - **If INP > 500ms (poor)** → Run full INP debugging workflow: 1. **webperf-interaction:Interactions.js** (list all interactions) 2. **webperf-interaction:Input-Latency-Breakdown.js** (phase breakdown) 3. **webperf-interaction:Long-Animation-Frames.js** (blocking frames) 4. **webperf-interaction:Long-Animation-Frames-Script-Attribution.js** (culprit scripts) - **If specific interaction type is slow (e.g., keyboard)** → Focus analysis on that interaction type - **After INP data is collected** → call `getINPDetails()` for the full sorted interaction list (useful for identifying patterns across multiple slow interactions) ### Error Recovery - **If any script returns `status: "error"`** → Check if the page has finished loading: - If early in load: wait and re-run the script - If page is an SPA: user may need to navigate to the target route first - **If LCP.js / LCP-Subparts.js returns `status: "error"`** → Tell the user: "LCP data is not available yet. Please ensure the page has fully loaded, then run the analysis again." - **If INP.js `getINP()` returns `status: "error"`** → The `getDataFn: "getINP"` field signals the agent can retry after user interaction. Prompt the user to click, type, or scroll, then call `getINP()` again. ### Cross-Skill Triggers > **Context fork note:** This skill runs with `context: fork`. Cross-skill triggers below are **recommendations to report back to the parent agent**, not direct calls this subagent can execute. When a cross-skill trigger fires, tell the user which skill and script to run next. Scripts marked _(pending)_ are not yet available — skip them and note the limitation. These triggers recommend using snippets from other skills: #### From LCP to Loading Skill - **If LCP > 2.5s and TTFB phase is dominant** → Use **webperf-loading** skill: - TTFB.js, TTFB-Sub-Parts.js, Service-Worker-Analysis.js - **If LCP image is lazy-loaded** → Use **webperf-loading** skill: - Find-Above-The-Fold-Lazy-Loaded-Images.js - **If LCP has no fetchpriority** → Use **webperf-loading** skill: - Priority-Hints-Audit.js #### From CLS to Loading Skill - **If CLS caused by fonts** → Use **webperf-loading** skill: - Fonts-Preloaded-Loaded-and-used-above-the-fold.js - Resource-Hints-Validation.js (for font preload) - **If CLS caused by images** → Use **webperf-media** skill: - Image-Element-Audit.js (check for width/height attributes) #### From INP to Interaction Skill - **If INP > 200ms** → Use **webperf-interaction** skill for full debugging: - Interactions.js, Input-Latency-Breakdown.js - Long-Animation-Frames.js, Long-Animation-Frames-Script-Attribution.js - LongTask.js (if pre-interaction blocking suspected) #### From LCP/INP to Interaction Skill - **If render delay or interaction delay is high** → Use **webperf-interaction** skill: - Long-Animation-Frames.js (main thread blocking) - LongTask.js (long tasks delaying rendering) ### Multi-Metric Correlation When multiple CWV metrics are poor, prioritize investigation: - **If LCP > 2.5s AND CLS > 0.1** → Likely shared root cause: 1. Check for late-loading content pushing LCP element 2. Run LCP-Trail.js to see LCP candidate changes 3. Run Layout-Shift-Loading-and-Interaction.js to correlate timing - **If LCP > 2.5s AND INP > 200ms** → Main thread congestion: 1. Run Long-Animation-Frames.js 2. Run webperf-loading:Script-Loading.js 3. Run webperf-loading:JS-Execution-Time-Breakdown.js - **If CLS > 0.1 AND INP > 200ms** → Layout thrashing or interaction-triggered shifts: 1. Run Layout-Shift-Loading-and-Interaction.js 2. Run Interactions.js 3. Check if shifts occur during/after interactions ## References - `references/snippets.md` — Descriptions and thresholds for each script - `references/schema.md` — Return value schema for interpreting script output