Fontconfig to make Emacs recognise semilight Cascadia fonts as such

80-cascadia-semilight.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>

  <!-- Place into an fcconfig directory (e.g. /etc/fonts/conf.d/) -->
  <!-- Run 'fc-conflist' to see where your fontconfig is looking. -->
  <!-- Run 'fc-cache -f' after dropping this file.                -->

  <description>Adjust weight of Cascadia fonts for Emacs</description>

  <match target="scan">
    <test target="font" name="family" ignore-blanks="true">
      <string>Cascadia Mono</string>
    </test>
    <test target="font" name="weight">
      <const>semilight</const>
    </test>
    <edit name="weight">
      <const>book</const>
    </edit>
  </match>

  <match target="scan">
    <test target="font" name="family" ignore-blanks="true">
      <string>Cascadia Mono PL</string>
    </test>
    <test target="font" name="weight">
      <const>semilight</const>
    </test>
    <edit name="weight">
      <const>book</const>
    </edit>
  </match>

  <match target="scan">
    <test target="font" name="family" ignore-blanks="true">
      <string>Cascadia Code</string>
    </test>
    <test target="font" name="weight">
      <const>semilight</const>
    </test>
    <edit name="weight">
      <const>book</const>
    </edit>
  </match>

  <match target="scan">
    <test target="font" name="family" ignore-blanks="true">
      <string>Cascadia Code PL</string>
    </test>
    <test target="font" name="weight">
      <const>semilight</const>
    </test>
    <edit name="weight">
      <const>book</const>
    </edit>
  </match>

</fontconfig>