HTML email / HTML newsletter client compatibility checklist

There are endless arguments of should HTML email be used or not. This article only deal with a single problem: if you want to author an HTML email, for example, to satisfy a corporate visual identity requirement for newsletters, what difference does it make to author an HTML email than to author a HTML webpage?

The difference is you need to stay compatible with a different set of clients. So in the end you need a client compatibility checklist to check against. Here is it.

There are several differences between HTML email and HTML web page that are commmon to all email clients:

http-equiv attribute doesn't work. Because HTTP protocol is not used.
<title> element is not found in use in any email client.

Yet there are a lot of differences between each email client on how they support HTML/CSS features.

Comparison of HTML/CSS features supported by various email clients
	Client	Gmail	Microsoft Outlook 2003	Mozilla Thunderbird 3	Mozilla Thunderbird 3 as authoring client
CSS	Class attribute	NG	OK	OK	Can be Prepared, only editable on selectable elements, they are: images <img> tables <table> table <td>
	Styles defined by style attribute	OK	OK	OK	Ditto
	Stylesheets embeded by <style> tag.	NG (dropped)	OK	OK	Can be Prepared, not editable
	Stylesheets referred by <link> element	NG	OK	Ask User	Loads or not depends on previous user choice
Images	<BASE> element		OK	OK	Ditto
	Images attached in email	OK	OK	OK	By default images are attached in email
	Images referred by external link in HTML		OK	Ask User	Loads or not depends on previous user choice
	Images embeded (using `data:` URI) in HTML		NG	OK	Can be prepared turns to attached image if sent.
	Images referred in CSS			Same as images referred in HTML	NG Even if prepared, during editing image references are converted to anchors (`<a...>`).

Can be Prepared, not editable: Can be prepared by the web designer who speaks HTML/CSS in advance, but not editable by users.; The method I use involve a few steps, briefly speaking, I keep one HTML email template in a Local Folder called "Templates" and edit the folder file directly, usually located in "~/.thunderbird/*/Mail/Local Folders/Templates" on my computer. This method perhaps does't work when there are more than one templates in the folder.
OK: This feature is supported.
NG: The feature is not supported or works badly. Don't use it.
(empty): Empty cell means the feature is not tested.

I am pretty sorry for this website lack of commenting feature, please write me an email if you offer new compatibility information or want to correct mine.