> By default, a <formula> is assumed to contain character data which
> is not validated in any way: <formula notation="TeX"> a<b </formula>
> It seems incompatible with well-formedness, isn't it?
Yup. That's an egregious error. I will fix it later today. Thanks!
(If you really want the TeX "a<b", you'd need to use "a<b", of