"Using markup (and style sheets) where possible rather than images [...] promotes accessibility for the following reasons:
Search engines can use text information." 
Even if the accessibility for search engines is not really relevant for mobile browsers you may assume that images are either not loaded or not well visible in a mobile browser. Therefore, the whole textual content in your pages should be coded only with markups regardless of its correspondence with your corporate design. It is better to be able to read some text in a standard font and font size than to see only a placeholder for an unloaded image.
The W3C suggests in  using the Mathematical Markup Language (MathML) or TeX for complex mathematical equations. Since many mobile browsers only support a subset of HTML 3.2 these suggestions cannot be applied.
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann