All the icons automatically injected into the bookdown GitBook header should have alt/hover text. Here is the header from the bookdown book itself: https://bookdown.org/yihui/bookdown/

Some of the elements have aria-label, but that is for assistive technologies, not most browsers / viewers. Bookdown should additionally give those elements title to describe the buttons for folks who hover to understand the underlying functionality.

Consider this example from the RStudio Connect documentation; many viewers may not understand that the "A" is for text resizing and the PDF logo is a PDF logo (Adobe Acrobat?) that triggers a file download.

This issue is present in many bookdown versions, including bookdown 0.9.

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