document.write('\
\
  <meta name="description" content="Noticias semanales sobre Anime.">\
  <meta name="author" content="Rubén - twitter.com/cont3mpo">\
  <meta name="viewport" content="width=device-width, shrink-to-fit=no, initial-scale=1">\
  <link rel="shortcut icon" type="image/png" href="img/favicon-newsletter.png">\
  <link rel="stylesheet" href="css/normalize.css">\
  <link rel="stylesheet" href="css/style.css">\
\
');

document.write('\
\
  <meta name="description" content="Noticias semanales sobre Anime.">\
  <meta name="author" content="Rubén - twitter.com/cont3mpo">\
  <meta name="viewport" content="width=device-width, shrink-to-fit=no, initial-scale=1">\
  <link rel="shortcut icon" type="image/png" href="img/favicon-newsletter.png">\
  <link rel="preload stylesheet" href="css/normalize.css">\
  <link rel="preload stylesheet" href="css/style.css">\
\
');

Update head-newsletter.js