Drop AVIF files saved from the web or exported from a phone. Each is written as JPEG at matched quality. Transparent AVIFs wait for a background colour, because JPEG cannot hold transparency.
Why AVIF needs converting
AVIF is the newest format a browser commonly serves, and much software has not caught up: older Windows photo viewers, office suites, many upload portals and some messaging apps refuse it. JPEG opens everywhere. The AVIF is decoded by this browser's own decoder — current Chrome, Edge, Firefox and Safari all have one; the empty converter shows whether this one does.
What changes
AVIF can store 10 or 12 bits per channel and wide-gamut colour. A browser decodes to 8 bits per channel in sRGB, so the JPEG is 8-bit sRGB; extra depth and any HDR rendition do not survive. JPEG is lossy, so the quality is searched until the JPEG measures visually identical to the decoded AVIF (0.990 at 512 px). Expect the JPEG to be larger than the AVIF — that is the cost of a format everything opens.
