The built-in kepub stylesheet has a line "* { -webkit-hyphenation: auto; }" to turn on hyphenation, but it is only used if the device's justification button is set to full justification. With this patch enabled hyphenation will always/never be turned on, regardless of justification button setting. (Note that if the publisher has set the hyphenation in the book's stylesheet then that will override the built-in stylesheet.)