What the quality slider changes
JPEG is lossy. A value of 100 is not a bit-perfect original — it is “very little extra loss.” WebpUndo maps the slider from 50 to 100 onto the browser’s toBlob('image/jpeg', quality) argument. The default of 85 is a practical still-photo setting: small enough for email, clean enough for a blog hero.
Drop below 70 only when a form has a hard size cap. Blocky skies and muddy type show up first. If the image is a screenshot or a poster with text, switch to PNG instead of cranking JPEG to 100.
Transparency becomes white
JPEG has no alpha channel. Before encode, the canvas is filled with white, then the WebP is drawn on top. Soft shadows and cutouts that looked correct on a checkerboard will sit on a white plate. If you need the holes in the picture, export PNG.
Why convert at all if WebP is already small?
Compatibility. Digital-asset systems from the 2010s, some print RIPs, and a surprising number of “upload your headshot” portals still sniff forimage/jpeg. A WebP that looks perfect in Chrome is not accepted. Converting locally is faster than arguing with the form and safer than using a random upload tool.
Color and generations
You are looking at pixels the browser already decoded. A second JPEG pass can add a little more ringing around edges. Keep quality high if this JPG might be compressed again by a CMS. If you still have the camera original, prefer that over any WebP you saved from a webpage.
Read how WebP works if you want the container and codec picture, or the no-upload guide if you are evaluating the tool for a workplace.