When you send HTML emails you can't used style tags but instead you
have to put inline style attributes on every element. So from this:
<html>
<style type="text/css">
h1 { border:1px solid black }
p { color:red;}
p::first-letter { float:left; }
</style>
<h1 style="font-weight:bolder">Peter</h1>
<p>Hej</p>
</html>
You want this:
<html>
<h1 style="font-weight:bolder; border:1px solid black">Peter</h1>
<p style="{color:red} ::first-letter{float:left}">Hej</p>
</html>
premailer does this. It parses an HTML page, looks up style blocks
and parses the CSS. It then uses the lxml.html parser to modify the
DOM tree of the page accordingly.
Another thing premailer can do for you is to turn relative URLs (e.g.
"/some/page.html" into "http://www.peterbe.com/some/page.html"). It
does this to all href and src attributes that don't have a ://
part in it. For example, turning this:
<html>
<body>
<a href="/">Home</a>
<a href="page.html">Page</a>
<a href="http://crosstips.org">External</a>
<img src="/folder/">Folder</a>
</body>
</html>
Into this:
<html>
<body>
<a href="http://www.peterbe.com/">Home</a>
<a href="http://www.peterbe.com/page.html">Page</a>
<a href="http://crosstips.org">External</a>
<img src="http://www.peterbe.com/folder/">Folder</a>
</body>
</html>
Certain HTML attributes are also created on the HTML if the CSS
contains any ones that are easily translated into HTML attributes. For
example, if you have this CSS: td { background-color:#eee; } then
this is transformed into style="background-color:#eee" AND as an
HTML attribute bgcolor="#eee".
Having these extra attributes basically as a "back up" for really shit email clients that can't even take the style attributes. A lot of professional HTML newsletters such as Amazon's use this.