For fitting an image to a limit like "under N KB" — application forms, message boards, upload forms. This page opens the compressor with the size target already switched on. Add your images and type the target in KB or MB.
Once a target is set, each image is re-compressed up to six times at lowering quality, looking for the highest quality whose result fits under the target. The quality you set is the ceiling of that search, so a result far below the target is never pushed back above it.
The resolution is not reduced to reach the target, so squeezing a large photo into a very small size can fall short; the smallest result reached is then marked as missing the target. Everything runs inside your browser, and files are never uploaded.