After the first visual test, some macros used in chem environment seem not to be displayed correctly
From MMLmhchemTest
Indices to fix:
28 fix harpoons appearance longrightleftharpoons
29 fix harpoons appearance longRightleftharpoons
63 bonds not displayed correctly in ref and TexVC
64 bonds not displayed correctly in ref and TexVC
Correct rendering can be seen here:
https://en.wikipedia.beta.wmflabs.org/wiki/Help:MathTestCustomChem