This test checks symmetric stretching. For that purpose, it uses fractions with empty numerator denominator in order to determine the math axis (as the vertical center of the mfrac). The test does not use any math web font. This used to work well with the old implementation and without math font when the num/denum are shifted symmetrically with respect to the math axis. After bug 201242, the reference is the baseline so that no longer works. An alternative would be to use the middle of a <mo>−</mo> instead but unfortunately it is not necessarily the same as the fallback for AxisHeight used when math fonts are absent, since WebKit just does xheight/2 rather than the measuring the glyph. An when a math font is present, AxisHeight is not necessarily xheight/2. So I guess the test should be rewritten with a web font instead. It seems it is very similar to https://w3c-test.org/mathml/presentation-markup/operators/mo-axis-height-1.html, maybe we just need to export more cases to WPT.