is_front_page() returns true if the user is on the page or page of posts that is set to the front page on Settings->Reading->Your homepage displays, so if you set
about us as the front page then this conditional will only be true if showing the about us page.
is_home() return true when on the posts list page, This is usually the page that shows the latest 10 posts.
If the settings under Your homepage displays are left at default then the home page will return true for both
is_home()to do this.
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