In some particular case, \not followed by a character command will result in the correct generation of the negated character (for example \not\in), which is good.

In more general case (for example \not\operatorname{R} on https://en.wikipedia.org/wiki/Help:Displaying_a_formula#Logic), Mathoid will generate something like

<math xmlns="http://www.w3.org/1998/Math/MathML">

<mrow class="MJX-TeXAtom-REL"> <mpadded width="0"> <mtext>⧸</mtext> </mpadded> </mrow> <mi mathvariant="normal">R</mi>

</math>

to display a slash bar on top of the character. This is strongly discouraged by the MathML spec:

http://www.w3.org/TR/MathML/chapter3.html#presm.warnspacing

In this particular case, there is an appropriate way to encode slash: use menclose with a strike notation. This is supported at least by NVDA+MathPlayer and Orca dev version, and I opened https://bugs.webkit.org/show_bug.cgi?id=146543 for VoiceOver.