[tannevaled]

Summary

Continuing round 199's methodology shift: instead of cold-grepping CSS/JS
feature names across sites, render real live pages through this engine's
own Fetch+RenderDocument with JSLog capturing every compile/runtime
error, then look at what actually fired.

• A sweep of 10 real sites (news, SaaS, framework-marketing homepages)
  found document.referrer.indexOf(hostname) throwing TypeError: Cannot read property 'indexOf' of undefined or null at trackPageview
  identically on both vuejs.org and astro.build — the same
  third-party script (cdn.usefathom.com/script.js) on two otherwise-
  unrelated sites, the same "one popular library, many sites break the
  same way" shape document.location (round 199, js: wire document.location to the same object as window.location #338) had.
• grep -rn referrer js/ (excluding tests) returned nothing at all — the
  property genuinely did not exist.
• Fixed: a new read-only accessor on document (js/dom.go, next to
  the existing hidden/visibilityState accessors) — always the empty
  string, per HTML Standard §7.8.1's own "no referring document" case,
  which this engine's single-URL fetch model always is.
• Verified end to end against both real live sites, before/after: the
  TypeError disappears from JSLog on both; vuejs.org's own separate,
  unrelated, already-documented dynamic-import() SyntaxError is
  untouched, confirming the fix's effect is precisely scoped.
• A second lead, not pursued this round: stripe.com's own bundle
  calls <canvas>.getContext(...), unimplemented. Larger in scope than a
  single property stub — flagged for a dedicated future round.
• Also confirmed, correctly out of scope: top-level await (bbc.com,
  nextjs.org) and dynamic import() (vuejs.org, svelte.dev) are genuine
  upstream goja parser gaps, no parser in this repo to patch.

Test plan

• go test ./... — all green, no coverage-gate regression
• js/js_test.go's new TestDocumentReferrerIsEmptyString — calls
  .indexOf() on the result directly, the real trigger's own exact
  shape
• Independently re-rendered both real live sites before/after: the
  TypeError line disappears, nothing else changes
• go build ./... && go vet ./... clean

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