One npm alias replaces TypeScript's two-engine DOM library

`Property 'startViewTransition' does not exist on type 'HTMLElement'` is a claim about how many engines have shipped the method, not about your browser ^[2]. TypeScript's web API types come from a generator whose policy is to emit only APIs supported by at least two browser engines ^[1]. Element-scoped view transitions, the Long Animation Frame API's `scripts` property and `fetchLater()` are all in Chrome, and all three are safe to use now as progressive enhancements, according to the post ^[5][3][4].

The replacement works by name. TypeScript resolves its DOM library by looking for `@typescript/lib-dom`, so the install line aliases the package to that specifier: `npm install --save-dev @typescript/lib-dom@npm:modern-web-types` ^[7]. On TypeScript 6 and newer you also set `"libReplacement": true` in `compilerOptions` ^[8]. The swap happens at lib resolution, which means it applies to every file the program compiles, with no per-file opt-in ^[18].

modern-web-types is built on TypeScript's own generator with the support threshold set to one engine ^[6]. After the swap the compiler accepts `fetchLater()`, and it no longer distinguishes an API two engines ship from one only Chrome ships. Whether the call site is guarded becomes a question for code review. Contributors to TypeScript-DOM-lib-generator describe the two-engine rule as a balance between early adoption and web compatibility, the post says ^[11]. Its author disagrees: "But to be honest, I'm not convinced that this policy actually promotes web compatibility," he wrote on philipwalton.com ^[12], and he does not think the rule has been a net positive for the open web ^[13].

The generation is automated. It uses the same TypeScript-DOM-lib-generator and the same w3c/webref data sources TypeScript itself uses, with the threshold lowered from two engines to one ^[9]. A weekly GitHub Actions job reruns the generator against the latest data, compares the output against the published version and opens a PR when it differs; once that PR is approved a new version publishes, and approval is the only manual step, though the post says it may be automated later ^[10].

The post includes a table counting the extra declarations the one-engine threshold adds across DOM and WebWorker, and points readers at the repo's `report.md` for the current list ^[15]. The row its author singles out is the one covering interfaces already in constant use: properties and methods missing from `Document`, `Element`, `Navigator` and `Request` ^[14]. "Before starting this project, I knew there were a lot of APIs that were not in TypeScript's official libraries, but even I was surprised to discover just how big that number was," he wrote ^[16].

For a project that already feature-detects before calling anything new, the swap deletes the hand-copied declarations the author says he carried between projects for years and costs one compiler option ^[17][8].