Prohibiting tabs after spaces is not enough. For example, Python rejects this code:

    if 1:
    <space>if 1:
    <tab>pass

because its indentation is invalid if tab width is 1. However, it accepts this code:

    if 1:
    <tab>if 1:
    <10 spaces>pass

despite its indentation being invalid if tab width is 10 or more.