See http://www.mathml-association.org/MathMLinHTML5/S2.html#SS2.SSS2 and http://www.mathml-association.org/MathMLinHTML5/S3.html#SS4.SSS1
The test for this is LayoutTests/imported/mathml-in-html5/mathml/presentation-markup/scripts/subsup-parameters-1.html LayoutTests/imported/w3c/web-platform-tests/mathml/presentation-markup/scripts/subsup-parameters-1.html