From Feedback:

You can consider using that invisible apply character (U+2061) after other function names with known arguments (such as \sin, \cos...). Some accessibility tools take advantage of it, I am told, but it may also be used to provide correct spacing from the MathML operator dictionary. In Chrome <mi>sin</mi><mi>a</mi> will show up with no added spacing as sina, which is not ideal.

as it also endorses InvisibleApply later on: https://w3c.github.io/mathml/#presm_invisibleops