Considering look & feel, some (fairly subjective) notes are below.
For comparison, various HTML pages:
(*) Note: Test file posted for this purpose only. It is based on, but not equivalent to, RFC 5234.
1) the fonts
2) The gray box for the header
If there were 5 authors, the gray box would be big – 11 lines.
I like it better than [B], but I wonder if there is a better way to do the header.
3) Figures (background color)
[A] doesn't contain an example of a figure; curious to see if it does the same as [E] (box with a gray background) or [F] (box with light yellow background). I prefer the yellow. Paul says that he prefers no background colors at all for text-based figures and listings because even light colors makes it somewhat harder to read the text.
4) Accent colors
[A] uses green for the title; blue for links.
[E] uses red for title; red for links.
[F] uses red for title; blue for links.
I don't feel strongly, but [A] is fine.
5) Table of Contents (linked text)
[A] each number is an internal link *and* each section title is an internal link. I think it looks odd. (And they are not necessarily the same, e.g., http://greenbytes.de/tech/webdav/draft-reschke-xml2rfc-latest.html#elements vs. #rfc.section.2).
Paul prefers links on section titles because they are longer and therefore easier to click on.
Look at [XML] in section 5.1. Both the title and the URL are links to the same file. This seems redundant. Is it necessary that titles be links?
Aspects of [E] that have purposely been dropped?