Can someone point more specifically to where it says anything about
variation selectors? This pointer is to the table of contents/overview...
Well, at http://www.usefulcontent.org/docs/manuals/REC-MathML2-20010221/chapter6.html#chars_mathmlchars I find:
<< This specification of MathML makes use of some characters that are not part of Unicode 3.0 but which have been proposed to the Unicode Technical Committee (UTC), and thus for inclusion in ISO 10646. They are presently expected to be in the revisions Unicode 3.1 and 3.2. (For more detail about this see Section 6.4.4 [Status of Character Encodings].)
While the process of review and adoption by UTC and ISO/IEC of the characters of special interest to mathematics and MathML is largely complete (Unicode Work in Progress) there remains the possibility of some further modification of the lists of characters accepted, of the code assignments for those adopted, or of the names given them by Unicode. To make sure any possible corrections to relevant standards are taken into account, and for the latest character tables and font information, see the W3C Math Working Group home page and the Unicode site. >>
This indicates we are dealing with a fairly old and tentative recommendation.
Later:
<< 6.3.5 Variant Mathematical Characters
Unicode attempts to avoid having several character codes for simple font variants. For a code point to be assigned there should be more than a nuance in glyphs to be recorded. To record variants worth noting there is a special character proposed for Unicode 3.2, U+FE00 (VARIATION SELECTOR-1), which acts as a postfix modifier. However the legally allowed combinations with this variation selector are restricted to a list recorded as part of Unicode. The VARIATION SELECTOR-1 character may only be applied to the characters listed here. The resulting combination is not regarded by Unicode as a separate character, but a variation on the base character. Unicode aware systems may render the combination as the base if the available fonts do not support the variant glyph shape. >>
At the bottom of the page:
<< The additions accepted are expected to be published as an amendment to [ISO/IEC 10646-1], and to become part of Unicode 3.2. It can therefore be expected that almost all of the characters in this category will finally be accepted, and encoded at the current code points. It is possible that a small number of characters may be renamed, moved, or less likely, ultimately rejected. There is even a handful of characters which have still Private Use Area codes in our tables; even these it is hoped will finally be fully recognized. Until final acceptance, implementers and users of MathML are using these characters and code points at their own risk. Entities and the mathvariant attribute are used to avoid that risk. >>
Variants that are part of the recommendation appear at http://www.usefulcontent.org/docs/manuals/REC-MathML2-20010221/variants.html including the slashed zero glyph.
It would seem that the slashed zero variant was either not actually included in the variants eventually proposed or was not accepted or (barely possible) was somehow lost and forgotten about.
But the recommendation is quite clear about the tentative nature of its presentation.
Jim Allan
