[toSum.tentative.html]
  [Converting a CSSNumericValue to a sum with invalid units throws SyntaxError]
    expected: FAIL

  [Converting a CSSNumericValue with an invalid sum value to a sum throws TypeError]
    expected: FAIL

  [Converting a CSSNumericValue with compound units to a sum throws TypeError]
    expected: FAIL

  [Converting a CSSNumericValue with leftover units to a sum throws TypeError]
    expected: FAIL

  [Converting CSSNumericValue to a sum with its own unit returns itself]
    expected: FAIL

  [Converting CSSNumericValue to a sum with no arguments returns all the units in sorted order]
    expected: FAIL

  [Converting CSSNumericValue to a sum with a relative unit converts correctly]
    expected: FAIL

  [Converting CSSNumericValue to a sum containing extra units returns zero for those units]
    expected: FAIL

  [CSSNumericValue.toSum converts greedily]
    expected: FAIL