chromium/chrome/browser/resources/side_panel/read_anything/menus/letter_spacing_menu.html

<simple-action-menu
    id="menu"
    label="$i18n{letterSpacingTitle}"
    event-name="[[toolbarEventEnum_.LETTER_SPACING]]"
    menu-items="[[options_]]"
    current-selected-index="[[restoredLetterSpacingIndex_(settingsPrefs)]]"
    on-letter-spacing-change="onLetterSpacingChange_"
    initial-count="3">
</simple-action-menu>