Just quick-posting this for my own reference to tumblr because I haven’t tested on any other browsers but firefox, BUT… this works:

span[style^=""]{
    color: inherit !important;
    font-size: inherit !important;
}

Add any other properties you like. This essentially tells the browser that if it finds any spans with a style applied, to ignore and inherit from the stylesheet (pretty much).

It’s tough to account for every possible element you could put an inline style on, but with CMS’ particularly Wordpress, the standards is to wrap it in a span.

Why? - because it’s annoying when I see a nice pretty design of mine get torn to shreds by florescent colors in an inline style. I only do this on very rare occasions when I have a client who I know is likely to mark up their content in weird ways and I also know the integrity of their site is important and I want to make sure things stay clean and classy.

It’s also not a bad idea for working with a social network site where you’re allowed to add to the existing stylesheet but can’t actually alter it. And worse, when they put inline styles in, either because they don’t want you messing with that part of your profile, or more common, they just don’t know about web standards. :)