IE9 uses DirectWrite to render text, and other browsers do not (except for Firefox 4+). That is the reason for the slightly different size of the text between the two browsers. There is no way to make the text the same size.
Breaking the Poetry Code - The future of poetry e-books, and why it’s not what you think. Some difficulties in converting printed poetry to e-books (semi-applicable to the web).
“Most e-readers use the ePub format, which generally strips text of most of its design and formatting,” explained Rachel Berchten, poetry and poetics editor at the University of California Press, in an e-mail. “For poems that are visual in nature, this is problematic.”
All the aspects of the poem—including irregular line breaks, indentations, and spaces (essentially, all the nuances that make a poem a poem)—must be described in a language the e-reader understands. So the markup language describes what the text is—the title, the epigraph, the text body of the poem, and so on—and the cascading style sheet (otherwise known as the CSS) tells the e-reader how to display what the markup language describes. The problem for ePub, the Kindle, and poetry is that the markup language doesn’t have the vocabulary to describe the minutiae of poetry.