And here is why not implementing MathML was a bad idea for the ePub format.
Unless the _only_ fractions are those for which there are accepted entities (quarters, halves and thirds) or are using an OpenType font which you can embed (and if the ePub will only be viewed on readers which support embedded fonts) and which has superscript and subscript numeral forms, the only guaranteed option is to convert the fractions to SVG graphics and place them in the text.
William
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William Adams
senior graphic designer
Fry Communications
Sphinx of black quartz, judge my vow.