chromium/chrome/browser/resources/compose/compose.html

<!DOCTYPE html>
<html dir="$i18n{textdirection}" lang="$i18n{language}">
  <head>
    <meta charset="utf-8">
    <title>Compose</title>
    <link rel="stylesheet" href="//resources/css/text_defaults_md.css">
    <link rel="stylesheet" href="//theme/colors.css?sets=ui,chrome">
    <style>
      html, body {
        cursor: default;
        margin: 0;
        min-width: fit-content;
        overflow: hidden;
      }

      html {
        background: var(--color-compose-dialog-background);
      }
    </style>
  </head>
  <body>
    <script type="module" src="app.js"></script>
    <compose-app></compose-app>
  </body>
</html>