FWIW I'm guessing that the epub has been converted via Calibre since there are references in the html
I downloaded a 'fresh' copy of what might be your book ...
http://www.gutenberg.org/ebooks/10031
... which has not been converted, but just opening in the Calibre editor there is no italics around the poems
Maybe you just need to retrieve a fresh copy as a starting point, i.e. one that never has been Calibre-converted