I much prefer the actual default browser styles. However, I believe that they should be customizable by the user without too much difficulty; I am not too concerned if the default setting is different than what I want it if the browser is designed well enough that the user can easily change the default CSS codes. I agree what they say about "reading length"; it should come down to preference. Fortunately, in Firefox (at least the old version I have) it is easy enough to disable CSS (ALT+V Y N); however, some things don't work very well with this, so maybe it might be better to use a mixture of ARIA and HTML to set up the default styles, although to get any reasonable working might need meta-CSS (which is my own idea; note that only the end-user has permission to apply meta-CSS code and the document author cannot).
The "XHTML Club" (linked from Ugly Duck) uses the following CSS code:
img{max-width:100%;}
pre{overflow:auto;}
This rather minimal CSS is what seems to me reasonable additions to the default CSS (although the user should still be allowed to customize them, and there should still be a command to display a picture by itself and be able to zoom it to full size in that case).