There are plans for JPEG XL to support transparency. From Draft Call for Proposals on Next-Generation Image Coding (JPEG XL):
1.2 Scope
The next-generation image coding activity aims to develop an image
coding standard that offers:
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- Features for web applications, such as support for alpha channel coding and animated image sequences.
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4.2 Compressed bitstream requirements
Submissions shall cover at least the core requirements, and are encouraged to cover desirable requirements as well.
Core requirements
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Support for alpha channel / transparency coding
