The most important one is the positioning of some elements in these files. Now there is a cascade of <ul> tags used, to get some indent. XHTML validators don't like that at all
Another problem is the use of font types and sizes in the files, eg for foot notes or using courier for specific things like the emoticons.
Since there is already a stylesheet for the documentation, it would IMHO be a good idea to use the same file for both purposes.
In the doc files, positioning is often hardcoded with e.g. <p style="margin-left: 40px;">
For usability on mobiles and pda's, that also could better be moved to a stylesheet.
Is this idea good enough to start working on a CSS-frame for the helpfiles?
Stylesheet for help files should be, they are real strangers within Moodle .