From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

When you compose an HTML message, you can insert HTML tags or edit existing HTML tags in the message:

1. Optionally, to edit existing HTML tags, select part of the message, or select the entire message (for example, by pressing Ctrl+A or by choosing Edit -> Select All).
2. Choose: Insert -> HTML...
3. Edit the HTML code as desired.
4. Press Insert.

See below for extensions that provide other ways to edit HTML tags.

Note:  Some people who receive your message might not see the effect of any changes that you make to HTML tags. For example, Thunderbird users might choose to display messages as plain text or as simple HTML, and webmail clients might change your HTML in various ways.

Advanced

An HTML message is a standard HTML document in two parts—the head and the body. Only the body contains HTML for display, so in Thunderbird you can only edit the body.

If you want to edit the head for some special purpose, then you can use an extension to edit it. However, even if you do edit the head, some mail clients that receive your message might ignore it.

For example if you plan to add a style tag containing CSS, the standard place for this in in the head, but adding it in the body might work better. See below for an external link to more information about CSS support in various mail clients.

Extensions

• EditHTML is for editing both the head and the body of the message. It adds a menu choice for this in the Format menu.
• External editor is for editing the head, the body and some message headers in an external text editor. It adds a toolbar button for this, and a menu choice in the Tools menu. However version 0.7.3 of this extension has a bug that prevents changes to the head from being added to the message correctly.
• Edit HTML Source is only for editing the body. It adds a menu choice in the View menu. Version 0.0.1 is not compatible with Thunderbird 1.5 and later releases.

See also

• Mail content types is an overview of HTML and plain text messages in Thunderbird
• Dialog too small or too large includes a way to make the Insert HTML dialog larger by default

External links

• CSS support in webmail Analysis of how some well-known webmail clients handle HTML messages
• HTML Mozilla's technical information on HTML