According to Met, By default, the theme uses a native system font stack. She made this choice for a couple of reasons: ❶ It will not take any extra load time. Met wants to keep this theme simple and fast. ❷ Keeping in mind the large scale of usability as it will be the default theme of WordPress Core. Actually, the native system font stack is pretty typographically “neutral” — none of the fonts are super opinionated so that the theme can be used broadly across different types of sites. ❸ Using just the one font stack, without loading additional font files, also makes it easier for folks to customize or create a child theme for Twenty Twenty-One. They want this theme to be a teaching tool and an outlet for your creativity.