I’ve just completed a new I6 extension, EffetsDeTexte (in French, but I provided some documentation and function names in English for English speakers).
It does every text effect I could think of : text styles, colors (and combinations of styles and colors), centered text, printing text after some time has elapsed or a key has been pressed, printing text after some time has elapsed disregarding key presses, printing text progressively/slowly, and printing a string ‘letter by letter’ (one letter is printed for every keystroke from the player). It’s bi-platform (except for text in color, which can’t be done in Glulx), and it checks whether the interpreter supports the effect you want to use (for instance, Parchment doesn’t support time, so you can’t use a ‘print slowly’ effect without making your game unplayable online). All that for about 4K (and I have hopes to trim that down).
Let me know if you have any feedback on usability, bugs or suggestions !