II. Technical Challenges (Why This Is Not "Just Add onerror")

Dimension	Challenge
Semantics	Must satisfy: try initial URL first, circuit mainly constrains fallback hosts, unmatched resources must not change semantics (still giveup, but different from entering the fallback chain).
Vite	Rewriting dynamic import too early breaks __vitePreload/__vite__mapDeps and async component CSS; literal dynamic import replacement must happen after the bundle is finalized.
Webpack	Global error and __webpack_require__.l injection can both see the same failure — without DOM markers, double retry; async CSS chunks (mini-css-extract, experiments.css, non-j loaders in __webpack_require__.f) reject promises — Observer replacing <link> alone leaves Promise.all short-circuited: JS fallback succeeds but lazy load still throws ChunkLoadError.
ESM	Failed module record cache causes "looks like fallback but Network is flat"; same URL + query strategy must be consistent across all entry points.
Config vs reality	When Vite base/publicPath and rule base diverge, opening the rewrite gate can rewrite async chunks to the wrong CDN — product and engineering misjudgment; needs a shouldRewriteUrls gate (compare after ensureTrailingSlash).
Legacy	SystemJS and Observer without mutually exclusive URL registration → double fallback or missed handling.

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