GIF's background is opacity or not opacity.

    apng, webp's background can be alpha value, they are supported by all current web browsers.
    webp's performance is better than apng.

    You can gather some png images with alpha channel to an apng image using python ffmpeg lib. Using follow command:
    ffmpeg -i img_%d.png -plays 0 -delay 10 img_out.apng

    The half-opacity animation image file format can be used in all web browsers, only can be apng or webp.

    For the tasks I will join in for my income, I bought another app: Pixelmator Pro with severial hundreds yuan.