Reconsider default Accept values for images

[valenting]

What is the issue with the Fetch Standard?

It seems that the default value for the Accept header was decided in #43
It's possible things have changed a bit since then.
I did a quick check of what are the default accept headers for images and icons and got this:

Chrome
icon: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8
image: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8

Firefox:
icon: image/avif,image/jxl,image/webp,*/*
image: image/avif,image/jxl,image/webp,*/*

(Note that image/avif and image/jxl might not be present for all users)

Safari:
icon: */*
image: image/webp,image/avif,image/jxl,image/heic,image/heic-sequence,video/*;q=0.8,image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5

@zcorpan suggested it might be interesting to see if we want to reconsider the default values.

fetch/fetch.bs

Line 4391 in 8dd73db

<dd>`<code>image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5</code>`

"image"
image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5

It's especially interesting that Firefox and Chrome don't have image/png anymore.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1711622#c13 for initial report.