look into: https://github.com/microsoft/pxt

• and all of the chai/mocha TDD related dependencies

• the content and style could be a simple prototype (doesn't need to look nice) – please take a look at the nova-editor and see how some of the visuals are displayed on top of the code.
• correct positioning: to be displayed correctly at the issue's line and character (column)
• I think you want to make sure that the positioning gets updated if the code moves (it might simply mean deleting the previous visual error/warning message, and running the detector again)

• use Finite State Machines to parse each line of Python code
• offer more relevant suggestions on top of the six existing syntax checkers
  • i.e., upon typing for i in, the program should suggest a missing iterable, instead of a missing colon

Parse a line of Python code such that existing/expected tokens can be identified