Third-party reachability

Some readers cannot reach some third parties: YouTube is blocked in mainland China, and a school, an office or another country's network blocks hosts of its own. When a page embeds a third party that has an alternative, the question it needs answered is not where the reader is or what language they read, but whether this reader's network reaches that host right now. So the page asks the network.

How it works

flowchart LR
  Need["A page needs the third party"] --> Probe["checkIsReachable<br/>no-cors request, bounded by a timeout"]
  Probe -- "any HTTP answer in time" --> Primary["The primary embed"]
  Probe -- "network error, block or timeout" --> Alternative["The alternative that works everywhere"]

• The probe is a real request to the host the embed loads from. checkIsReachable sends a no-cors request, which any origin answers without a CORS header, so its response is never read: any HTTP answer, a 404 included, means the network reaches the host. A blocked host fails the request outright, or swallows it until AbortSignal.timeout aborts it.
• Anything short of an answer is a no. A failure, a timeout and a request an ad blocker refuses all pick the alternative, so the probe fails toward the choice that plays rather than toward a broken frame.
• The probe never stands between a reader and the content. It starts as early as the page can send it, so its answer is in before the choice is made, and the choice reads whatever answer is in at that moment — a probe still out is a no. No reader waits for it, on an open network or a blocked one, and no order of preference by region is needed, since that only matters when somebody waits on the first guess.

Why not the reader's language or location

• Language is a preference, not a place. Accept-Language and navigator.languages say what a reader reads: a reader in Shanghai with an English browser, and one in Singapore reading Simplified Chinese, would both be sent the wrong way. Game text resolves a language from them (matchGameLanguage) and nothing else does.
• Location is not the network either. Geolocating the IP address needs a lookup service or a CDN header the host does not send, and still misreads a reader on a VPN, a reader abroad, and every network that blocks a host without being in a country that does.

Key files