Browser-based vs server-based image conversion
A converter can process a selected image in the browser or upload it to a remote service. Those designs have different data flows.
What browser-based conversion means here
This tool processes selected image bytes with code running in the browser and creates a downloadable JPEG there. The original source file is not changed. That statement concerns the conversion operation, not every request involved in visiting a website.
What the release check observes
For the two supported conversions and the unsupported failure case, release smoke observes conversion-time fetch, XHR, WebSocket, EventSource, ping, and non-GET/HEAD requests. It found none. The check distinguishes those from ordinary page, static-asset, Cloudflare delivery, and Cloudflare Web Analytics requests described in the privacy policy. This evidence supports the stated conversion path for those checked cases; it is not a claim that a web visit makes no network requests.
What server-based conversion changes
A server-based service generally sends the selected image to a remote system for processing. That can help with formats or workloads a browser cannot handle, but it also adds transfer and retention questions. This site is delivered through Cloudflare; see Cloudflare's privacy policy for its delivery-related processing.
Limits remain
Browser support, memory, image size, and HEIF variants vary. Local processing does not make every HEIC file compatible. The tool gives a clear error for an unsupported case rather than producing a substitute image.