Most graphical email clients support HTML email, and many default to it. Many of these clients include both a GUI editor for composing HTML emails and a rendering engine for displaying received HTML emails. Long URLs can be linked to without being broken into multiple pieces, and text is wrapped to fit the width of the user agent's viewport, instead of uniformly breaking each line at 78 characters (defined in RFC 5322, which was necessary on older text terminals). It allows in-line inclusion of tables, as well as diagrams or mathematical formulae as images, which are otherwise difficult to convey (typically using ASCII art).
Since its conception, a number of people have vocally opposed all HTML email (and even MIME itself), for a variety of reasons. The ASCII Ribbon Campaign is an internet phenomenon advocating that email should continue to be sent in Human Readable ASCII text format. While still considered inappropriate in many newsgroup postings and mailing lists, its adoption for personal and business mail has only increased over time. Some of those who strongly opposed it when it first came out now see it as mostly harmless.
According to surveys by online marketing companies, adoption of HTML-capable email clients is now nearly universal, with less than 3% reporting that they use text-only clients. A smaller number, though still the majority, prefer it over plain text.
Email software that complies with RFC 2822 is only required to support plain text, not HTML formatting. Sending HTML formatted emails can therefore lead to problems if the recipient's email client does not support it. In the worst case, the recipient will see the HTML code instead of the intended message.
Among those email clients that do support HTML, some do not render it consistently with W3C specifications, and many HTML emails are not compliant either, which may cause rendering or delivery problems, especially for users of Gmail.
In particular, the
<head> tag, which is used to house CSS style rules for an entire HTML document, is not well supported, sometimes stripped entirely, causing in-line style declarations to be the de facto standard, even though they are not optimal from a semantic web point of view. Although workarounds have been developed, this has caused no shortage of frustration among newsletter developers, spawning the grassroots Email Standards Project, which grades email clients on their rendering of an acid test, inspired by those of the Web Standards Project, and lobbies developers to improve their products. To persuade Google to improve rendering in Gmail, for instance, they published a video montage of grimacing web developers, resulting in attention from an employee.
|Clients||Result (as of)|
|AOL Webmail||Solid support (13 July 2011)|
|Apple iPhone||Solid support (13 July 2011)|
|Apple iPod Touch|
|Apple Mail||Solid support (28 November 2007)|
|Apple MobileMe||Solid support (15 August 2008)|
Eudora OSE codenamed "Penelope"
|Solid support (28 November 2007)|
|Microsoft Entourage||Solid support (28 November 2007)|
|Mozilla Thunderbird||Solid support (28 November 2007)|
|Windows Live Mail||Solid support (28 November 2007)|
|Windows Mail||Solid support (28 November 2007)|
|Yahoo! Mail Beta||Solid support (8 July 2011)|
|Windows Live Hotmail||Some improvement recommended (8 July 2011)|
|Google Gmail||Improvement recommended (13 July 2011)|
|Lotus Notes 8||Improvement recommended (28 November 2007)|
|Microsoft Outlook 2007||Improvement recommended (28 November 2007)|
Some senders may excessively rely upon large, colorful, or distracting fonts, making messages more difficult to read. For those especially bothered by this formatting, some user agents make it possible for the reader to partially override the formatting (for instance, Mozilla Thunderbird allows specifying a minimum font size); however, these capabilities are not globally available. Further, the difference in optical appearance between the sender and the reader can help to differentiate the author of each section, improving readability.
Many email servers are configured to automatically generate a plain text version of a message and send it along with the HTML version, to ensure that it can be read even by text-only email clients, using the
Content-Type: multipart/alternative, as specified in RFC 1521. The message itself is of type
multipart/alternative, and contains two parts, the first of type
text/plain, which is read by text-only clients, and the second with
text/html, which is read by HTML-capable clients. The plain text version may be missing important formatting information, however. (For example, a mathematical equation may lose a superscript and take on an entirely new meaning.)
The order of the parts is significant. RFC1341 states that: In general, user agents that compose multipart/alternative entities should place the body parts in increasing order of preference, that is, with the preferred format last. For multipart emails with html and plain-text versions, that means listing the plain-text version first and the html version after it, otherwise the client may default to showing the plain-text version even though an html version is available.
HTML email is larger than plain text. Even if no special formatting is used, there will be the overhead from the tags used in a minimal HTML document, and if formatting is heavily used it may be much higher. Multi-part messages, with duplicate copies of the same content in different formats, increase the size even further. The plain text section of a multi-part message can be retrieved by itself, though, using IMAP's FETCH command.
Although the difference in download time between plain text and mixed message mail (which can be a factor of ten or more) was of concern in the 1990s (when most users were accessing email servers through slow modems), on a modern connection the difference is negligible for most people, especially when compared to images, music files, or other common attachments.
HTML allows a link to be displayed as arbitrary text, so that rather than displaying the full URL, a link may show only part of it or simply a user-friendly target name. This can be used in phishing attacks, in which users are fooled into believing that a link points to the website of an authoritative source (such as a bank), visiting it, and unintentionally revealing personal details (like bank account numbers) to a scammer.
If an email contains web bugs (inline content from an external server, such as a picture), the server can alert a third party that the email has been opened. This is a potential privacy risk, revealing that an email address is real (so that it can be targeted in the future) and revealing when the message was read. For this reason, some email clients do not load external images until requested to by the user.
During periods of increased network threats, the US Department of Defense converts all incoming HTML email to text email.
The multipart type is intended to show the same content in different ways, but this is sometimes abused; some email spam takes advantage of the format to trick spam filters into believing that the message is legitimate. They do this by including innocuous content in the text part of the message and putting the spam in the HTML part (that which is displayed to the user).
- Configuring Mail Clients to Send Plain ASCII Text — E-mail client programs
- HTML Email: Whenever Possible, Turn It Off!
- HTML Email: The Poll (Scot Hacker, originator of the much-linked-to Why HTML in E-Mail is a Bad Idea discusses how his feelings have changed since the 1990s)
- "Email Marketing Statistics and Metrics". Emaillabs.com. Retrieved 2012-06-24.
- "Real-World Email Client Usage: The Hard Data". Clickz.com. Retrieved 2014-05-21.
- Dialect <http://dialect.ca/>. "Premailer: make CSS inline for HTML e-mail". Premailer.dialect.ca. Retrieved 2012-06-24.
- "Why we need standards support in HTML email". Campaign Monitor. Retrieved 2012-06-24.
- "The 2008 Gmail Appeal | Email Standards Project". Email-standards.org. Retrieved 2012-06-24.
- Shobe, Matt (2004-10-12). "A pretty fair argument against HTML Email". Burningdoor.com. Retrieved 2012-06-24.
- RFC 1521 7.2.3. The Multipart/alternative subtype
- "TN1010-11-2: Multipart/Alternative — Gracefully handling HTML-phobic email clients." (PDF). Retrieved 2012-06-24.
- "Sending HTML and Plain Text E-Mail Simultaneously". Wilsonweb.com. 2000-04-28. Retrieved 2012-06-24.
- "RFC1341 Section 7.2 The Multipart Content-Type". Retrieved 2014-07-15.
- "Do we really want to send web pages in e-mail?". Dsv.su.se. Retrieved 2012-06-24.
- HTML Email — Still Evil?
- "DOD bars use of HTML e-mail, Outlook Web Access". fcw.com. Retrieved 2015-06-23.