title: Why does 'x-www-form-urlencoded' begin with 'x-www', when other standard content types do not?
body: <p>I understand that in the past, it was standard for custom headers names to use the prefix "X-" (I'm aware it no longer is considered standard to do this), but I've been unable to find if there is any relationship between this naming convention and the value ("application/x-www-form-urlencoded"). Did it start out as a custom content-type value that was later adopted or something?</p>
  
  <p>I found <a href="https://www.w3.org/Protocols/rfc1341/4_Content-Type.html" rel="noreferrer">this</a> link here, which certainly was interesting, but have been unable to find the answer to my question.</p>
  
  <p>Does anybody know the reason this prefix was chosen, and what it signifies?</p>