Start with the image, not a universal number
Quality scales are encoder-specific. A value such as 80 is not a standardized promise of a particular file size or perceptual quality across browsers and libraries. Use it as a starting point, then compare the actual output.
Photographs often tolerate moderate lossy compression better than typography, UI screenshots, line art or images with broad smooth gradients. Those images can show halos, jagged edges or banding earlier, so use higher quality or a lossless format when needed.
A quick visual check
Inspect a crop at 100% zoom for fine texture and edges, then view the image at its real page size. Look especially at hair, leaves, small text, thin diagonals and subtle color transitions. A tiny artifact at full resolution may be invisible in context or obvious on a large hero image.
Also test the dimensions. Reducing an image from a very large source to its rendered size can save more bytes than pushing quality lower. Keep the original source and compare candidate outputs side by side.
Frequently asked questions
Is quality 100 lossless?
Not necessarily. Quality 100 generally means a high lossy quality request; only a specific lossless encoding mode guarantees lossless coding.