title: not used characters from ANSI characters set
body: <p>I'am developing a small programming language together with an IDE.</p>
  
  <p>The ANSI character set states the subset of unused characters. Here is the complete list: 0x7F, 0x81, 0x8D, 0x8F, 0x90, 0x9D</p>
  
  <p>I'd like to use some of them for an invisible code markup, so am wondering how they got printed in different environments. Can I assume they are always a whitespace, or some editors will take the honor to replace them with something like '?' or grey rectangle?</p>
  
  <p>Thank you, Dmitry</p>