why some thin character roboto ?

Question

ı use "roboto" and "san-serif" ,normal text "A.Ş" error text -->

Answer 1

Use fonts from googleapis and it will work

   body {
      font-family: 'Roboto', sans-serif;
      font-size: 48px;
      font-weight: bold;
    }

 <link href='http://fonts.googleapis.com/css?family=Roboto' rel='stylesheet' type='text/css'>
 
 <p>A.Ş</p>

Answer 2

It's because your font doesn't have that character (Ş), so it uses the default font that has it. And if san-serif isn't a typo here, you should change it to sans-serif.