I loved AbiWord's file format. It was literally a basic html file, so I could confidently store anything in it and know I could read it later, much easier than any ODF.
Images are also PNG, JPEG and SVG. Formulas are MathML.
Styling uses the same properties as CSS, but the are written in XML instead of using the CSS syntax.
The ODF Essentials book is good introduction to the format. http://books.evc-cit.info/odbook/book.html