Add Lighthouse testing infrastructure for performance and quality audits

[Copilot]

This PR implements Lighthouse testing based on the nodejs.org implementation to provide automated performance, accessibility, SEO, and best practices audits for the GitHub Pages site.

What's Added

Lighthouse Configuration (.lighthouserc.json)

• Runs 3 audits per URL for reliable metrics
• Tests performance, accessibility, best practices, and SEO
• Configured with appropriate score thresholds and CI-friendly Chrome flags
• Uploads reports to temporary public storage for easy sharing

GitHub Actions Workflow (.github/workflows/lighthouse.yml)

• Automatically runs after successful deployments to main branch
• Tests both the homepage and experiment template pages
• Uploads comprehensive HTML reports as workflow artifacts (30-day retention)
• Includes manual trigger option for on-demand testing
• Uses SHA-pinned actions for security

Local Development Support

• Added @lhci/cli dependency for local lighthouse testing
• New yarn test-lighthouse script for development workflow
• Updated documentation with complete testing instructions
• Added .lighthouseci/ to .gitignore to exclude generated reports

Usage

Automated Testing: Lighthouse audits run automatically after each deployment to main branch. Download the HTML reports from the workflow artifacts.

Local Testing:

# Start the development server
bundle exec jekyll serve

# In another terminal, run lighthouse
yarn test-lighthouse

The generated HTML reports provide detailed insights into:

• Performance metrics and optimization opportunities
• Accessibility compliance and improvements needed
• SEO best practices and recommendations
• Code quality and modern web standards adherence

Fixes #156.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• accounts.google.com
  • Triggering command: /opt/google/chrome/chrome --type=utility --utility-sub-type=network.mojom.NetworkService --lang=en-US --service-sandbox-type=none --no-sandbox --disable-dev-shm-usage --use-angle=swiftshader-webgl --mute-audio --crashpad-handler-pid=4575 --enable-crash-reporter=, --noerrdialogs --user-data-dir=/tmp/lighthouse.eWlZtJI --change-stack-guard-on-fork=enable --shared-files=v8_context_snapshot_data:100 --field-trial-handle=3,i,5706456788514042299,14311980995302636841,262144 --disable-features=AutofillServerCommunication,CalculateNativeWinOcclusion,CertificateTransparencyComponentUpdater,DialMediaRouteProvider,InterestFeedContentSuggestions,MediaRouter,OptimizationHints,PaintHolding,PrivacySandboxSettings4,Translate --variations-seed-version (dns block)
  • Triggering command: /opt/google/chrome/chrome --disable-features=Translate,OptimizationHints,MediaRouter,DialMediaRouteProvider,CalculateNativeWinOcclusion,InterestFeedContentSuggestions,CertificateTransparencyComponentUpdater,AutofillServerCommunication,PrivacySandboxSettings4 --disable-extensions --disable-component-extensions-with-REDACTED-pages --disable-REDACTED-networking --disable-component-update --disable-client-side-phishing-detection --disable-sync --metrics-recording-only --disable-default-apps --mute-audio --no-default-browser-check --no-first-run --disable-REDACTEDing-occluded-windows --disable-renderer-REDACTEDing --disable-REDACTED-timer-throttling --disable-ipc-flooding-protection --password-store=basic --use-mock-keychain --force-fieldtrials=*BackgroundTracing/default/ --disable-hang-monitor --disable-prompt-on-repost --disable-domain-reliability --propagate-iph-for-testing --remote-debugging-port=46005 --disable-setuid-sandbox --user-data-dir=/tmp/lighthouse.eWlZtJI --no-sandbox --disable-dev-shm-usage --headless=new --noerrdialogs --ozone-platform=headless --ozone-override-screen-size=800,600 --use-angle=swiftshader-webgl about:blank (dns block)
• android.clients.google.com
  • Triggering command: /opt/google/chrome/chrome --type=utility --utility-sub-type=network.mojom.NetworkService --lang=en-US --service-sandbox-type=none --no-sandbox --disable-dev-shm-usage --use-angle=swiftshader-webgl --mute-audio --crashpad-handler-pid=4575 --enable-crash-reporter=, --noerrdialogs --user-data-dir=/tmp/lighthouse.eWlZtJI --change-stack-guard-on-fork=enable --shared-files=v8_context_snapshot_data:100 --field-trial-handle=3,i,5706456788514042299,14311980995302636841,262144 --disable-features=AutofillServerCommunication,CalculateNativeWinOcclusion,CertificateTransparencyComponentUpdater,DialMediaRouteProvider,InterestFeedContentSuggestions,MediaRouter,OptimizationHints,PaintHolding,PrivacySandboxSettings4,Translate --variations-seed-version (dns block)
  • Triggering command: /opt/google/chrome/chrome --disable-features=Translate,OptimizationHints,MediaRouter,DialMediaRouteProvider,CalculateNativeWinOcclusion,InterestFeedContentSuggestions,CertificateTransparencyComponentUpdater,AutofillServerCommunication,PrivacySandboxSettings4 --disable-extensions --disable-component-extensions-with-REDACTED-pages --disable-REDACTED-networking --disable-component-update --disable-client-side-phishing-detection --disable-sync --metrics-recording-only --disable-default-apps --mute-audio --no-default-browser-check --no-first-run --disable-REDACTEDing-occluded-windows --disable-renderer-REDACTEDing --disable-REDACTED-timer-throttling --disable-ipc-flooding-protection --password-store=basic --use-mock-keychain --force-fieldtrials=*BackgroundTracing/default/ --disable-hang-monitor --disable-prompt-on-repost --disable-domain-reliability --propagate-iph-for-testing --remote-debugging-port=46005 --disable-setuid-sandbox --user-data-dir=/tmp/lighthouse.eWlZtJI --no-sandbox --disable-dev-shm-usage --headless=new --noerrdialogs --ozone-platform=headless --ozone-override-screen-size=800,600 --use-angle=swiftshader-webgl about:blank (dns block)
• clients2.google.com
  • Triggering command: /opt/google/chrome/chrome --type=utility --utility-sub-type=network.mojom.NetworkService --lang=en-US --service-sandbox-type=none --no-sandbox --disable-dev-shm-usage --use-angle=swiftshader-webgl --mute-audio --crashpad-handler-pid=4575 --enable-crash-reporter=, --noerrdialogs --user-data-dir=/tmp/lighthouse.eWlZtJI --change-stack-guard-on-fork=enable --shared-files=v8_context_snapshot_data:100 --field-trial-handle=3,i,5706456788514042299,14311980995302636841,262144 --disable-features=AutofillServerCommunication,CalculateNativeWinOcclusion,CertificateTransparencyComponentUpdater,DialMediaRouteProvider,InterestFeedContentSuggestions,MediaRouter,OptimizationHints,PaintHolding,PrivacySandboxSettings4,Translate --variations-seed-version (dns block)
  • Triggering command: /opt/google/chrome/chrome --disable-features=Translate,OptimizationHints,MediaRouter,DialMediaRouteProvider,CalculateNativeWinOcclusion,InterestFeedContentSuggestions,CertificateTransparencyComponentUpdater,AutofillServerCommunication,PrivacySandboxSettings4 --disable-extensions --disable-component-extensions-with-REDACTED-pages --disable-REDACTED-networking --disable-component-update --disable-client-side-phishing-detection --disable-sync --metrics-recording-only --disable-default-apps --mute-audio --no-default-browser-check --no-first-run --disable-REDACTEDing-occluded-windows --disable-renderer-REDACTEDing --disable-REDACTED-timer-throttling --disable-ipc-flooding-protection --password-store=basic --use-mock-keychain --force-fieldtrials=*BackgroundTracing/default/ --disable-hang-monitor --disable-prompt-on-repost --disable-domain-reliability --propagate-iph-for-testing --remote-debugging-port=46005 --disable-setuid-sandbox --user-data-dir=/tmp/lighthouse.eWlZtJI --no-sandbox --disable-dev-shm-usage --headless=new --noerrdialogs --ozone-platform=headless --ozone-override-screen-size=800,600 --use-angle=swiftshader-webgl about:blank (dns block)
• safebrowsingohttpgateway.googleapis.com
  • Triggering command: /opt/google/chrome/chrome --type=utility --utility-sub-type=network.mojom.NetworkService --lang=en-US --service-sandbox-type=none --no-sandbox --disable-dev-shm-usage --use-angle=swiftshader-webgl --mute-audio --crashpad-handler-pid=4575 --enable-crash-reporter=, --noerrdialogs --user-data-dir=/tmp/lighthouse.eWlZtJI --change-stack-guard-on-fork=enable --shared-files=v8_context_snapshot_data:100 --field-trial-handle=3,i,5706456788514042299,14311980995302636841,262144 --disable-features=AutofillServerCommunication,CalculateNativeWinOcclusion,CertificateTransparencyComponentUpdater,DialMediaRouteProvider,InterestFeedContentSuggestions,MediaRouter,OptimizationHints,PaintHolding,PrivacySandboxSettings4,Translate --variations-seed-version (dns block)
  • Triggering command: /opt/google/chrome/chrome --disable-features=Translate,OptimizationHints,MediaRouter,DialMediaRouteProvider,CalculateNativeWinOcclusion,InterestFeedContentSuggestions,CertificateTransparencyComponentUpdater,AutofillServerCommunication,PrivacySandboxSettings4 --disable-extensions --disable-component-extensions-with-REDACTED-pages --disable-REDACTED-networking --disable-component-update --disable-client-side-phishing-detection --disable-sync --metrics-recording-only --disable-default-apps --mute-audio --no-default-browser-check --no-first-run --disable-REDACTEDing-occluded-windows --disable-renderer-REDACTEDing --disable-REDACTED-timer-throttling --disable-ipc-flooding-protection --password-store=basic --use-mock-keychain --force-fieldtrials=*BackgroundTracing/default/ --disable-hang-monitor --disable-prompt-on-repost --disable-domain-reliability --propagate-iph-for-testing --remote-debugging-port=46005 --disable-setuid-sandbox --user-data-dir=/tmp/lighthouse.eWlZtJI --no-sandbox --disable-dev-shm-usage --headless=new --noerrdialogs --ozone-platform=headless --ozone-override-screen-size=800,600 --use-angle=swiftshader-webgl about:blank (dns block)
• www.acls.net
  • Triggering command: /usr/local/bin/node scripts/generate-sitemap.mjs (dns block)
• www.google.com
  • Triggering command: /opt/google/chrome/chrome --type=utility --utility-sub-type=network.mojom.NetworkService --lang=en-US --service-sandbox-type=none --no-sandbox --disable-dev-shm-usage --use-angle=swiftshader-webgl --mute-audio --crashpad-handler-pid=4575 --enable-crash-reporter=, --noerrdialogs --user-data-dir=/tmp/lighthouse.eWlZtJI --change-stack-guard-on-fork=enable --shared-files=v8_context_snapshot_data:100 --field-trial-handle=3,i,5706456788514042299,14311980995302636841,262144 --disable-features=AutofillServerCommunication,CalculateNativeWinOcclusion,CertificateTransparencyComponentUpdater,DialMediaRouteProvider,InterestFeedContentSuggestions,MediaRouter,OptimizationHints,PaintHolding,PrivacySandboxSettings4,Translate --variations-seed-version (dns block)
  • Triggering command: /opt/google/chrome/chrome --disable-features=Translate,OptimizationHints,MediaRouter,DialMediaRouteProvider,CalculateNativeWinOcclusion,InterestFeedContentSuggestions,CertificateTransparencyComponentUpdater,AutofillServerCommunication,PrivacySandboxSettings4 --disable-extensions --disable-component-extensions-with-REDACTED-pages --disable-REDACTED-networking --disable-component-update --disable-client-side-phishing-detection --disable-sync --metrics-recording-only --disable-default-apps --mute-audio --no-default-browser-check --no-first-run --disable-REDACTEDing-occluded-windows --disable-renderer-REDACTEDing --disable-REDACTED-timer-throttling --disable-ipc-flooding-protection --password-store=basic --use-mock-keychain --force-fieldtrials=*BackgroundTracing/default/ --disable-hang-monitor --disable-prompt-on-repost --disable-domain-reliability --propagate-iph-for-testing --remote-debugging-port=46005 --disable-setuid-sandbox --user-data-dir=/tmp/lighthouse.eWlZtJI --no-sandbox --disable-dev-shm-usage --headless=new --noerrdialogs --ozone-platform=headless --ozone-override-screen-size=800,600 --use-angle=swiftshader-webgl about:blank (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[fulldecent]

prefer #158