Purdue Online Writing Lab (OWL)
National Genealogical Society | Guidelines for Publishing Web Pages on the Internet
Web Style Guide
American Psychological Association (APA) Style
   A Complete Resource for Writing and Publishing in the Social and Behavioral Sciences
   Is referenced a lot.
  In-house style - MozillaZine Knowledge Base
  
  Some common issues
  Prefered  Not preferd
  e-mail     Email
  yoko-ono   yoko_ono - Should I Use a Hyphen or Underscore in URL
  yoko-ono   Yoko-Ono  -  uppercase - URL Structure: Lower case VS Upper case - Stack Overflow
    Note: Wikipedia:Manual of Style requires the first letter to be capatilized.
   and they use Thomas_Jefferson which violates the above rules.
   

Wikipedia:Manual of Style - Wikipedia, the free encyclopedia

Style guide - Wikipedia, the free encyclopedia
Wikinews:Style guide - Wikinews, the free news source

W3C Manual of Style
Style Guide for Online Hypertext
www.w3.org/Provider/Style/All.html

Web Education community group style guide - Web Education Community 

How to write Knowledge Base articles | Mozilla Support -Support Document Guide
In-house style - MozillaZine Knowledge Base