For those not familiar with working with Cascading Style Sheets for use by Microsoft Office product, some of the rules of the road can prove cryptic at best. Case in point is the ” series of embedded styles. These can sometimes appear to conflict with other styles in the same declaration, for example:
<span style='font-size:11.0pt;mso-bidi-font-size:10.0pt;color:navy'>
The font-size is a standard CSS declaration, but the next declaration is particular to Microsoft:
The “mso” part is, of course, the abbreviation for Microsoft Office (since Microsoft almost always abbreviates its name as “ms” in code).  But what does “bidi” mean? I hunted everywhere, but had to resort to deep Googling to dig up the answer from the Windows Internet Explorer Blog. In a post dedicated to the issue of bidirectional text behavior in MSIE 6 vs. 7, the IE gurus explain:
When text is presented in horizontal lines, most scripts display characters from left to right. However, there are several languages (such as Arabic, Divehi, Hebrew and Syriac) where the natural ordering of horizontal text in display is right to left. Ambiguities can arise in determining the ordering of character display when text flows in two directions (hence Bidirectional) is present.
So the “bidi” means “bidirectional” and tells the Office-based application that declaration affects the display/print of bidirectional fonts, such as those for Arabic or Hebrew.
Therefore, the “mso-bidi-font-size:12.0pt” declaration means “Microsoft Office Bidirectional Font Size:12 point ” style and allows the setting of a 12 point font size in the event of a bidirectional font appearing within the bounds of the declaration
Hopefully I just saved some of you a bit of Googling.