HTML email: Difference between revisions

Revision as of 07:26, 24 December 2022

Revision as of 08:22, 27 December 2022

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{{Short description|Type of email}}

{{Use dmy dates|date=JanuaryDecember 20132022}}

{{POV|talk=POV|date=December 2021}}

'''HTML email''' is the use of a [[subset]] of [[HTML]] to provide formatting and [[semantic web|semantic]] markup capabilities in [[email]] that are not available with [[plain text]]:<ref>{{Cite web|title = Text Email vs HTML Email – The Pros and Cons {{!}} Thunder Mailer – Mass Emailing Software|url = http://www.thundermailer.com/text-email-vs-html-email-the-pros-and-cons/|website = www.thundermailer.com|access-date = 2016-01-30}}</ref> Text can be linked without displaying a [[URL]], or breaking long URLs into multiple pieces. Text is wrapped to fit the width of the viewing window, rather than uniformly breaking each line at 78 characters (defined in RFC 5322, which was necessary on older [[Data terminal#Text terminals|text terminals]]). It allows in-line inclusion of images, [[Table (information)|table]]s, as well as diagrams or [[mathematical formula]]e as images, which are otherwise difficult to convey (typically using [[ASCII art]]).

== Adoption ==

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Most graphical [[email client]]s 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.

Since its conception, a number of people have vocally opposed all HTML email (and even [[MIME]] itself), for a variety of reasons.<ref>[https://subversion.american.edu/aisaac/notes/htmlmail.htm HTML Email: Whenever Possible, Turn It Off!]</ref> For instance, the [[ASCII Ribbon Campaign]] advocated that all email should be sent in [[ASCII]] text format. The campaign was unsuccessful and was abandoned in 2013.<ref>{{Cite web |title=The Ascii Ribbon Campaign official homepage |url=http://www.asciiribbon.org/ |access-date=30 January 2016 |archive-url=https://web.archive.org/web/20100311081242/http://www.asciiribbon.org/ |archive-date=11 March 2010 |url-status=dead }}</ref><ref>{{Cite web|title = Shutdown of the ASCII ribbon campaign -– Pale Moon forum|url = http://forum.palemoon.org/viewtopic.php?f=4&t=2705|website = forum.palemoon.org|access-date = 2016-01-30|archive-url = https://web.archive.org/web/20160203102930/http://forum.palemoon.org/viewtopic.php?f=4&t=2705|archive-date = 3 February 2016|url-status = dead}}</ref> 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.<ref>[http://birdhouse.org/blog/2006/01/15/html-email-the-poll/ 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)</ref>

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.<ref>{{Cite web|title=Email Marketing Statistics and Metrics -– EmailLabs |url=http://www.emaillabs.com/tools/email-marketing-statistics.html |date=2007-03-29 |access-date=2016-01-30 |quote=HTML has nearly universal adoption among consumers: A Jupiter Research consumer survey found just 3% receive only text email. |url-status=dead |archiveurl=https://web.archive.org/web/20070329012457/http://www.emaillabs.com/tools/email-marketing-statistics.html |archivedate=29 March 2007 }}</ref> The majority of users prefer to receive HTML emails over plain text.<ref>{{Cite web|title = Real-World Email Client Usage: The Hard Data {{!}} ClickZ|url = https://www.clickz.com/clickz/column/2138714/real-world-email-client-usage-the-hard-data|website = www.clickz.com|access-date = 2016-01-30|last = Grossman|date = 2002-07-09|first = Edward|quote = Do you prefer receiving HTML or text email? HTML: 41.95%, Text: 31.52%, No preference: 26.53%}}</ref><ref>{{Cite web|title = The Science of Email Marketing|url = http://www.slideshare.net/HubSpot/the-science-of-email-marketng/32|website = www.slideshare.net|access-date = 2016-01-30|quote = In what format do you prefer to receive email messages from companies? HTML: 88%, Plain text: 12%}}</ref>

== Compatibility ==

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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.

In particular, the <code><nowiki><head></nowiki></code> 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|''de facto'' standard]], even though in-line style declarations are inefficient and fail to take good advantage of HTML's ability to separate style from content.{{cncitation needed|date=January 2015}} Although workarounds have been developed,<ref>{{cite web|author=Dialect <http://dialect.ca/> |url=http://premailer.dialect.ca/ |title=Premailer: make CSS inline for HTML e-mail |publisher=Premailer.dialect.ca |date= |accessdate=2012-06-24}}</ref> 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,<ref>{{cite web |url=http://www.email-standards.org/gmail-appeal |title=The 2008 Gmail Appeal &#124; Email Standards Project |publisher=Email-standards.org |accessdate=2012-06-24 |url-status=dead |archiveurl=https://web.archive.org/web/20120515030536/http://www.email-standards.org/gmail-appeal |archivedate=15 May 2012 |df=dmy-all }}</ref> resulting in attention from an employee.

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== Style ==

Some senders may excessively rely upon large, colorful, or distracting [[font]]s, making messages more difficult to read.<ref>{{cite web |last=Shobe |first=Matt |url=http://www.burningdoor.com/matt/archives/000782.html |title=A pretty fair argument against HTML Email |publisher=Burningdoor.com |date=2004-10-12 |accessdate=2012-06-24 |url-status=dead |archiveurl=https://web.archive.org/web/20120424084806/http://www.burningdoor.com/matt/archives/000782.html |archivedate=24 April 2012 |df=dmy-all }}</ref> For those especially bothered by this formatting, some [[user agent]]s 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.

== Multi-part formats ==

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 client]]s, using the <code>[[MIME content type|Content-Type]]: [[MIME#Alternative|multipart/alternative]]</code>, as specified in RFC 1521.<ref>[http://tools.ietf.org/html/rfc1521#section-7.2.3 RFC 1521 7.2.3. The Multipart/alternative subtype]</ref><ref>{{cite web|url=http://www.codestone.ltd.uk/software/docs/csmail/tn1010-11-2.pdf |title=TN1010-11-2: Multipart/Alternative —– Gracefully handling HTML-phobic email clients. |date= |accessdate=2012-06-24}}</ref><ref>{{cite web|url=http://www.wilsonweb.com/wmt5/html-email-multi.htm |title=Sending HTML and Plain Text E-Mail Simultaneously |publisher=Wilsonweb.com |date=2000-04-28 |accessdate=2012-06-24}}</ref> The message itself is of type <code>multipart/alternative</code>, and contains two parts, the first of type <code>text/plain</code>, which is read by text-only clients, and the second with <code>text/html</code>, 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.)

Many{{Citation needed|date=September 2009}} [[Electronic mailing list|mailing list]]s deliberately block HTML email, either stripping out the HTML part to just leave the plain text part or rejecting the entire message.{{Citation needed|date=September 2009}}

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.''<ref>{{cite web|url=http://www.w3.org/Protocols/rfc1341/7_2_Multipart.html|title=RFC1341 Section 7.2 The Multipart Content-Type|date= |accessdate=2014-07-15}}</ref> 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.

== Message size ==

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.<ref>{{cite web|url=http://dsv.su.se/jpalme/ietf/mhtml-discussion.html |title=Do we really want to send web pages in e-mail? |publisher=Dsv.su.se |date= |accessdate=2012-06-24}}</ref>

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 [[modem]]s), on a modern connection the difference is negligible for most people, especially when compared to images, music files, or other common attachments.<ref>[http://www.georgedillon.com/web/html_email_is_evil_still.shtml HTML Email —– Still Evil?]</ref>

== Security vulnerabilities ==

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HTML content requires email programs to use engines to parse, render and display the document. This can lead to more security vulnerabilities, denial of service or low performance on older computers.

During periods of increased network threats, the US Department of Defense converts all incoming HTML email to text email.<ref>{{cite web|url=http://fcw.com/articles/2006/12/22/dod-bars-use-of-html-email-outlook-web-access.aspx|title=DOD bars use of HTML e-mail, Outlook Web Access|publisher=fcw.com|date= |accessdate=2015-06-23}}</ref>

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 filter]]s 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).