When you edit a page with VE in IE, CSS styles are normalized, causing dirty diffs. This is because Internet Explorer very aggressively normalizes styles, to the point where we can't tell what the original value was without some nasty hackery.
In practice, this means that any time you save a page in IE, things like style="color: #ffffdd;" get normalized to style="color: rgb(255, 255, 221);", bgcolor="#FFFFDD" gets normalized to bgcolor="#ffffdd" and invalid CSS is removed. The former two are annoying dirty diffs, the latter is actual information loss (someone else can come along later and fix invalid CSS, but not if we remove it).
This normalization is a bug in Internet Explorer. I'm working on a workaround where I extract the original value using an XML parser and stash it so I can restore it later.