If you want to send HTML emails, you need to use a tool that let you write the content of the email yourself instead of relying on WYSIWYG editors.
The most part of mail clients doesn't allow this feature, mainly for security reasons.

With web email clients, such as Gmail, Yahoo or Hotmail, you can try to use the WebKit Inspector (in Safari and Chrome) or Firebug to edit the content of the email directly in the DOM, bypassing the interface restrictions. However, your code is likely to be sanitized by the application and it won't work as expected.

The best solution is to send the emails using a programming language (e.g. mail() in PHP or Pony in Ruby), or a mail service designed to handle HTML content such as MadMimi, Mailchimp, Campaign Monitor...

How can I write HTML and send as an email?
I am reading Judea Pearl's "Causality" (second edition 2009) and in section 1.1.5 Conditional Independence and Graphoids, he states:


  The following is a (partial) list of properties satisfied by the 
  conditional independence relation (X_||_Y | Z). 
  
  
  Symmetry: (X_||_ Y | Z) ==> (Y_||_X | Z).
  Decomposition: (X_||_ YW | Z)  ==> (X_||_Y | Z).
  Weak union: (X_||_ YW | Z)  ==> (X_||_Y | ZW). 
  Contraction: (X_||_ Y | Z) &amp; (X_||_ W | ZY) ==> (X_||_ YW | Z).
  Intersection: (X_||_  W | ZY) &amp; (X_||_  Y | ZW) (X_||_  YW | Z). 
  
  
  (Intersection is valid in strictly positive probability distributions.)
  
  (formula (1.28) given earlier in the publicatiob: [(X_||_ Y | Z) iff P(X | Y,Z ) = P(X | Z)  )


But what is an  "strictly positive distribution" in general terms, and what distinguishes a "strictly positive distribution" form a distribution that is not strictly positive?
