title: How to check if a highlight attribute is displayable on a terminal?
body: <p>I am writing a plugin and need to check if a highlight attribute (bold, italic,
  underline and so on) is actually displayable on a terminal.
  I tried to check it with termcap. For example of the underline,</p>
  
  <pre><code>!empty(&amp;t_us) &amp;&amp; !empty(&amp;t_ue)
  </code></pre>
  
  <p>becomes TRUE on xterm, and displayable.<br>
  However,<br>
  becomes TRUE on win32, but not displayable.<br>
  becomes FALSE on nvim, but displayable.</p>
  
  <p>Could someone have a good idea to check it correctly?</p>