That looks very inflexible for later edits, marking characters or words means a potentially cumbersome recounting if something is reworded or just if a typo is fixed.

Also, I get the second and subsequent number issue are the three locations to highlight, but what instruction does the first, -6:-3, give to the formatter in your example?

Not a big fan of byte/rune counting. Maybe regex. Maybe more verbosity, like:

  aftertext
    How much wood could a woodchuck chuck if a woodchuck could chuck wood?
    context How much wood
      italics wood
    context could a woodchuck
      italics wood
    context a woodchuck could
      italics wood
    context chuck wood
      italics wood

This is horrific, imagine changing your text and having to update the indices