I don't know if GPT-5 is an exception and is overcooked on XML specifically, but in general Markdown and XML seem to work about equally well for LLM inputs, the important part is just that they like hierarchical structured formats. The example on that page could probably be replaced with:

  ## Code Editing Rules

  ### Guiding Principles

  - Every component should be modular and reusable
  ...

  ### Frontend Stack Defaults

  - Styling: TailwindCSS