I know -- the problem is the syntax. You can't safely apply this to any document, because it may change the meaning of it. Such things should only exist as unicode characters or character entities.

I'm not sure your argument is valid. How is the decision made to convert x/y to fraction form? Is it white-space/punctuation+whitespace on either side? If that's the case, then: xxx/yyy will never become fraction form; however, x/y and x/y. (as in the end of a sentence) will.

The only way to guarantee it is to have an HTML tag to identify the fraction - <frac>11/63</frac> or similar.

But this does not work this way -- it seems it just blindly consumes char before and after / and converts if this fraction has a glyph in the font. Also, what you suggest is hard to implement since there are numerous localisation-dependent options what a whitespace/punctuation is and where to search it.