Match size and color needs
First reduce dimensions to the actual use size. If the image is a flat graphic with a small number of colors, a palette-based PNG optimizer can reduce the color table while preserving the appearance.
For photographs or mixed content, consider WebP if your clients support it. WebP supports alpha, but lossy encoding can change details and edges, so compare a crop at normal viewing size.
Why canvas re-encoding may not help
Browser canvas output is convenient for conversion but is not the same as a dedicated PNG optimization pipeline. It may keep the same pixels while choosing a less efficient compression strategy, so a PNG may stay the same size or become larger.
Avoid deleting the original until you have verified the dimensions, transparency and visual quality of the final file. Measure byte size directly after each change.
Frequently asked questions
Can PNG quality be lowered?
Not in the JPEG-style sense while remaining a lossless PNG. Reducing colors or dimensions changes pixel data; WebP offers a lossy alternative.