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.