This is an incredibly advanced option and the only real way to get highly specific formatting in an eBook is to do the high-level design by hand coding the features.
Some eReaders prefer readability over style. eReaders offer accessibility to readers who may need a different font, letter size, or spacing, by allowing the reader to adjust these elements on-demand. Badly formed or overly specific coding can make it difficult for readers to apply their personal settings.
Some devices and apps simply don’t have the capability to display certain code or fonts. Keeping the coding simple minimizes possible errors caused by this providing recommended guidelines, but allowing the programming interpret it in a native way.
Using too much, or overly complex coding can also inflate file sizes and often results in buggy, inconsistent, or even non-existent support in apps.
In the end, you run the risk of ending up with a brilliant looking eBook that maintains all of your setting but only works on a limited number of devices or apps.