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From an accessibility standpoint, a button with no visible text just gets read aloud as "button" by screen reader users, leaving them with no idea what it actually does.

Google PageSpeed Insights will flag this with a warning like the one shown above.
To fix this, add an aria-label to the button tag like so:
<button aria-label="ホームページへ移動"> <Button
variant="plain"
onClick={() => handleNavigate('/')}
aria-label="ホームページに移動"
>