As discussed on the 2023-01-05 telecon, this pull request changes the requirement to a recommendation due to the lack of support both for images and fallback alternative text in navigation document labels and headings.

I forgot that we added a caution box to the authoring specification about the lack of support, so we don't need to do anything more to warn authors about the usability issues non-text content can cause.

From an accessibility perspective, there are other problems with non-text content that the specification doesn't solve, so avoiding the content is probably the best solution (i.e., the alt text is not required to be presented if the non-text content is supported, which disadvantages non-sighted readers). Not presenting alt text when non-text content isn't supported disadvantages everyone equally.

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