When you use whitespace as markup, you need to make sure that it's visible whitespace, both on screen and printed. Otherwise it's not [ReadableMarkup].

For example, whitespace at the end of the lines is invisible.

Eight (or four) spaces is impossible to distinguish visually from a tab character -- thus spaces and tab should have the same meaning.

Spaces are hard to count -- thus any number of spaces run together should have the same meaning (i.e. be collapsed to one space).

Space near the wrapping point of a line is impossible to distinguish visually from a newline.