At Pomatio we are not very supportive of uploading it transparently, for two reasons:

1- Now there is dark mode / light mode in many browsers, and unless they are midtones, it is common that one of them (in this example on dark mode) does not look good:

dark mode

In this same example on light mode, it would look good:

Light mode

2- If we add access to the website to the desktop of our smartphone, something similar will happen:

Desktop of our smartphone

For this reason, we recommend uploading this file always with a background, either solid or gradient:

File always with background