Is that a Kindle issue?

You can download books in most browsers. I know Amazon have done things to make life difficult for other stores in the past.

I'd call it one of those middle-ground things:

• On the one hand, E Ink devices have a fairly known set of limitations, and it would be ridiculous for me to expect them to render the whole web well.

• On the other hand, it's good for website designs to consider the kind of devices employed by their users. Using a Kindle to access Gutenberg is likely less of an edge case than it would be for other sites, so it's worth the extra design work.

(Keep in mind that -- given my sibling comment -- this is all theoretical. The latest iteration of Gutenberg's site is much better than the previous version)