Re: [w3ctag/design-reviews] jxl Content-Encoding (#541)

It is proposed only to process `{0x0A, 0x04, 0x42, 0xD2, 0xD5, 0x4E}` case as Content-Encoding.
So, you are correct: only JPEG1 streams could be expressed in compressed form; arbitrary data could be "framed" (see `M.5.1`) as well.

`image/jxl` is already registered as provisional standard media type (see https://www.iana.org/assignments/provisional-standard-media-types/provisional-standard-media-types.txt). This media type corresponds to fully-fledged JPEG XL encoding.

Perhaps explainer lacks the motivation, why we want to add the lossless past of JPEG XL as Content-Encoding.
Going to update  explainer with the following information soon:

```
Pros
 + faster JPEG rendering of cached content: JPEG uses less CPU compared to modern image formats
 + compatibility: there are a lot of expensive cameras / smartphones that produce JPEGs; it will take at least 10 years to transition to the new format
 + seamless experience: when user saves the image (JPEG) it is guaranteed to be supported by existing software
 + lower deployment risks: Content-Encoding story is similar to Brotli, so it would not cause new risks
 + risk-free serving: no re-review is required for transcoded images, as those are guaranteed to be exactly the same
 + easier for webmasters: no image optimization stack is involved; nginx / Apache plugins will support the new encoding with the zero effort
 + exactly same pixels for legacy clients and new clients (both are decoding the same jpeg)
 + same behaviour: same progression

Contras
 - systems for image optimization / re-coding already exist (cloudinary, cloudflare, akamai, etc.): adding a new image format looks easier for those than adding new Content-Encoding
 - adding "save-as" in browsers would solve the "seamless experience" point (however it would unexpectedly for users loose transparency / animation / etc. features)
```



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