90-100
Visually lossless and wasteful for web delivery. File size climbs steeply for detail that is invisible on any screen. Reasonable only when the WebP is an intermediate that will be re-encoded again.
80-89
The working range for photography. At 85 almost nobody can pick the compressed version in a side-by-side at normal viewing size, and you are typically at a third of the equivalent JPG's weight.
70-79
Good for thumbnails, list images and anything displayed small. Artefacts exist but are invisible at the size the image is shown.
50-69
Visible softening in flat areas and around edges. Usable for background images and heavily blurred decorative graphics; not for content.
Below 50
Obvious degradation. Only worth it when a hard size cap leaves no alternative — and at that point, resizing the image is almost always the better lever.
Why WebP numbers differ from JPEG
The scales are encoder-specific and measure different things. A WebP at quality 75 typically looks closer to a JPEG at 85 while being smaller. Compare by eye at the size you will display, not by matching numbers.