[be5invis]

Let me count all the features that a proper rich text box should support:

  · Input methods, including...  
    · Composing IME  
    · Speech  
    · Handwriting  
    · Emoji picker  
  · Complex text rendering, including...  
    · Bidi  
    · Shaping  
    · Complex-script-aware editing  
    · Complex-script-aware find, replace, etc.  
  · Accessibility

And what they can choose to support:

  · Advanced typography, including...  
    · About line breaking...  
      · Hyphenation  
      · Optimized line break (Knuth-plass, etc.)  
    · About microtypography  
      · OpenType features (ligatures, etc.)  
      · OpenType variations  
      · Multiple master fonts  
      · Proper font fallback ← It is not a simple lookup-at-each-character process  
    · Advanced Middle East features, including...  
      · Kashida  
    · Advanced Far East features, including...  
      · Kinsoku Shori  
      · Auto space insertion  
      · Kumimoji  
      · Warichu  
      · Ruby  
    · Proper vertical layout, including...  
      · Yoko-in-Tate  
  · Inline objects and paragraph-like objects, including...  
    · Images  
    · Hyperlinks  
    · Math equations ← This is really hard.