I found this
<https://github.com/mathjax/MathJax-docs/wiki/Beveled-fraction-like-sfrac,-nicefrac-bfrac>
and
added this to my org file:

. . .
#+begin_html
<script type="text/x-mathjax-config">
        MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () { var
MML = MathJax.ElementJax.mml, TEX = MathJax.InputJax.TeX;
TEX.Definitions.macros.bfrac = "myBevelFraction"; TEX.Parse.Augment({
myBevelFraction: function (name) { var num = this.ParseArg(name), den =
this.ParseArg(name); this.Push(MML.mfrac(num,den).With({bevelled: true}));
} }); });
    </script>
#+end_html
. . .

then this:

$\pi = \bfrac{C}{d}$

has the slanted fraction look in HTML.

On Fri, Feb 19, 2016 at 9:35 PM, Marcin Borkowski <mbork@mbork.pl> wrote:

>
> On 2016-02-19, at 22:26, Lawrence Bottorff <borgauf@gmail.com> wrote:
>
> > I've got Latex's *nicefrac *working fine for a Latex export. I've put
> >
> > #+LaTeX_HEADER: \usepackage{nicefrac}
> >
> > at the top of my file, and this inlined
> >
> > $\pi = \nicefrac{C}{d}$
> >
> > But for an HTML export, it's not seeing it, giving back π=\nicefracCd. Is
> > there anything I can do to get my HTML export to handle nicefrac
> properly?
>
> Quick googling reveals that MathJax doesn't seem to support \nicefrac.
> If you don't need regular fractions (maybe you're writing a cookbook?),
> in a pinch you can say
>
> #+LaTeX_HEADER: \usepackage{nicefrac}   \let\frac=\nicefrac
>
> and just use \frac.
>
> > LB
>
> Best,
>
> --
> Marcin Borkowski
> http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
> Faculty of Mathematics and Computer Science
> Adam Mickiewicz University
>