Donald Knuth vs. W3C

On the one hand, the file plain.tex from Donald Knuth contains the line:

\def\sin{\mathop{\rm sin}\nolimits}

This means, sin, etc. should be treated as math operators, with extra space before and after.

On the other hand, the description of math identifiers (as distinguished from math operators) in MathML according to the World-Wide Web Consortium says:

Identifiers include function names such as "sin". Expressions such as "sin x" should be written using the character U+2061 (which also has the entity names ⁡ and ⁡) as shown below; see also the discussion of invisible operators in Section 3.2.5 Operator, Fence, Separator or Accent <mo>.

This means sin, etc. should be treated as though they were ordinary variables (except for being in roman type). The W3C did include the space after (in the form of ⁡, etc.) but not the space before.

I won't more than mention the annoyance of stretchy operators that aren't part of mfenced…