chromium/ash/webui/common/resources/office_fallback/main.html

<!doctype html>
<html dir="$i18n{textdirection}" lang="$i18n{language}">
  <head>
    <meta charset="utf-8">
    <link rel="stylesheet" href="chrome://resources/css/text_defaults_md.css">
    <link rel="stylesheet"
      href="chrome://resources/chromeos/colors/cros_styles.css">
    <link rel="stylesheet" href="chrome://theme/colors.css?sets=legacy,sys">
    <link rel="stylesheet" href="chrome://theme/typography.css">
    <style>
      body {
        background-color: var(--cros-bg-color-elevation-3);
        /** Use the same height as defined in C++*/
        height: 100vh;
        margin: 0;
      }
    </style>
    <script type="module" src="office_fallback_dialog.js"></script>
  </head>
  <body>
    <office-fallback></office-fallback>
  </body>
</html>