<!DOCTYPE html>
<html lang="ja">
  <head>
    <meta charset="UTF-8">
    <title>Slackker</title>
  </head>
  <body>
    <div id="app" />
  </body>
</html>

<!DOCTYPE html>
<html lang="ja">
  <head>
    <meta charset="UTF-8">
    <title>Slackker</title>
  </head>
  <body style='margin: 0'>
    <div id="app" />
  </body>
</html>

Modify style in html to zero margin
Modify style in html to zero margin
