There are probably better ways, and indeed, this method assumes there's no whitespace, and that the html is properly formed (it may also need tweaking because I typed it off the top of my head):
XElement tag = XElement.Parse("<span style="font-height:10px;">some text</span>");
XAttribute style = tag.Attribute("style").Value;
if (style != null)
{
string newTagString = tag.ToString().Replace(string.Format("style=\"{0}\"", style.ToString()), "");
tag = XElement.Parse(newTagString);
}