Your canvas.toBlob might be silently handing you a PNG

A user told me the .webp files my tool produced wouldn't open on their desktop. I opened one in a hex editor. First four bytes: 89 50 4E 47 . It was a PNG. With a .webp extension. The encoder wasn't broken. I had simply never checked whether the browser actually did what I asked. The spec says it's allowed to do this Here's the code. Nothing looks wrong with it: canvas . toBlob ( blob => { download ( blob , ' output.webp ' ); }, ' image/webp ' ); The callback fires. The blob isn't null. Its size looks reasonable. Everything succeeds — except it isn't WebP. This is not a bug. The HTML spec explicitly requires it: if the user agent doesn't support the requested type, it must create the file using the PNG format instead. No exception, no warning, no second argument telling you what happened. There's exactly one place that information exists — blob.type : canvas . toBlob ( blob => { console . log ( blob . type ); // iOS below 16.4: "image/png" }, ' image/webp ' ); toDataURL does the same thing, but at least there the fallback is visible to the naked eye, since the data URL literally starts with data:image/png;base64, . There is no capability query for this My first instinct was to special-case iOS. That falls apart quickly. Every browser on iOS is WebKit underneath, so "is this Safari" isn't a meaningful question. Embedded webviews inside apps track the system version in ways that don't always match the standalone browser. And a user can flip on "Request Desktop Website" and hand you a macOS user agent from an iPhone. More fundamentally: the user agent string answers "who are you" , and I need to know "can you encode WebP right now" . Between those two questions sit the engine version, OS version, host app, and build flags. Any mismatch in that chain and your lookup table lies to you. So I went looking for an official capability API. Media has them: MediaRecorder . isTypeSupported ( ' video/webm;codecs=vp9 ' ); // → boolean await navigator . mediaCapabilities . encoding