Where the TIFF requirement comes from
TIFF turns up in submission specs rather than in everyday use: commercial printers, journal and book publishers, stock libraries, museums, and digital preservation programs all standardize on it. The reasoning is that TIFF is lossless, decades old, thoroughly documented, and readable by professional software long after fashionable formats have come and gone. If a spec sheet says 300 dpi TIFF, a PNG will usually be bounced by whoever checks it, however identical the pixels are.
A lossless source into a lossless container
PNG and TIFF both store exact pixels, so this is one of the rare conversions with nothing to trade away: the TIFF holds precisely what the PNG held. Your file is decoded by the browser, redrawn at its original dimensions, and written out by a small open-source TIFF encoder running inside a Web Worker. The output is uncompressed, which is what most print and archive workflows expect, so it will typically be several times the size of the PNG. Nothing is resized and no resolution is invented — if a printer needs 300 dpi, that has to come from the pixel dimensions of the original.
Do not expect a preview
No browser can display TIFF, so the file you download will not open in a tab — it will simply prompt to save again. That is normal and does not mean the conversion failed. Windows Photos, macOS Preview, and every professional image editor open it without complaint, and if you want to eyeball the result in a browser you can convert it back on the TIFF to PNG page. As with everything here, the work is local: no account, no watermark, no limit on how many files you queue, and no copy of your image on anyone else’s machine.