Website does not set background color (but does set foreground)
Some websites do not explicitly set a foreground or background color. In this case, it's up to each user's browser to choose them. In Firefox, this can be set in the preferences, behind a "Manage Colors..." button. In addition to setting the fallback colors, the user may elect to override colors set by the website, but this issue is not about that case -- it is only about setting fallback colors, without overriding any choices from the website.
If the user sets their fallback colors to light-on-dark, for accessibility or any other reason, the Alpine website becomes partially illegible. This is because the site explicitly specifies (dark) foreground colors, while leaving the background color decision to the user's browser, despite having no knowledge of what that color is, or if the resulting combination will be legible.
This is especially notable for section titles.