Download as HTML and raw HTML doesn't display Math correctly anymore

The following document contains some math which is nicely displayed in the editor but when downloading as html the math becomes ugly and some things completely fail.

https://hackmd-ce.herokuapp.com/dRNM0WorQHuJCoFgm76iOQ

This is a regression that occurred both on hackmd.io and in codimd. Back in March when I wrote that document on hackmd.io the downloaded html displayed fine. Yet the exact same html that I downloaded in March doesn't render properly anymore now.

Edit: When I copy the head from the HTML of the online editor the math displays correctly again. So I guess it would be just a matter of updating the head used for the "Download HTML" function.

@jackycute You are working on hackmd.io, right? Maybe you could fix it there too? :P Btw, do you have a separate bug tracker for hackmd.io ?

Edit 2: https://github.com/hackmdio/codimd/issues/885 (inlining the resources) might have prevented the regression.