next.js/packages/next/src/next-devtools/dev-overlay/font/font-styles.tsx

import { css } from '../utils/css'
import { useInsertionEffect } from 'react'

export const FontStyles = () => {
  useInsertionEffect(() => {
    const style = document.createElement('style')
    style.textContent = css`
      /* latin-ext */
      @font-face {
        font-family: '__nextjs-Geist';
        font-style: normal;
        font-weight: 400 600;
        font-display: swap;
        src: url(/__nextjs_font/geist-latin-ext.woff2) format('woff2');
        unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7,
          U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F,
          U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F,
          U+A720-A7FF;
      }
      /* latin-ext */
      @font-face {
        font-family: '__nextjs-Geist Mono';
        font-style: normal;
        font-weight: 400 600;
        font-display: swap;
        src: url(/__nextjs_font/geist-mono-latin-ext.woff2) format('woff2');
        unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7,
          U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F,
          U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F,
          U+A720-A7FF;
      }
      /* latin */
      @font-face {
        font-family: '__nextjs-Geist';
        font-style: normal;
        font-weight: 400 600;
        font-display: swap;
        src: url(/__nextjs_font/geist-latin.woff2) format('woff2');
        unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6,
          U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122,
          U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
      }
      /* latin */
      @font-face {
        font-family: '__nextjs-Geist Mono';
        font-style: normal;
        font-weight: 400 600;
        font-display: swap;
        src: url(/__nextjs_font/geist-mono-latin.woff2) format('woff2');
        unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6,
          U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122,
          U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
      }
    `
    document.head.appendChild(style)

    return () => {
      document.head.removeChild(style)
    }
  }, [])

  return null
}