HTML e-mail
Encyclopedia
HTML email is the use of a subset
of HTML
(often ill-defined) to provide formatting and semantic
markup capabilities in email
that are not available with plain text
.
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.
HTML mail allows the sender to properly express quotations
(as in inline replying), headings, bulleted lists
, emphasized text
, subscripts and superscripts, and other visual and typographic cues to improve the readability and aesthetics of the message, as well as semantic information encoded within the message, such as the original author and Message-ID of a quote. Long URL
s 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 table
s, as well as diagrams or mathematical formulae as images, which are otherwise difficult to convey (typically using ASCII art
).
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.
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
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.
s, making messages more difficult to read. 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.
Many 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.
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.
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 bug
s (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).
Most email spam is sent in HTML for these reasons, so spam filters sometimes give higher spam scores to HTML messages.
Subset
In mathematics, especially in set theory, a set A is a subset of a set B if A is "contained" inside B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment...
of HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
(often ill-defined) to provide formatting and semantic
Semantic Web
The Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
markup capabilities in email
Email
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
that are not available with plain text
Plain text
In computing, plain text is the contents of an ordinary sequential file readable as textual material without much processing, usually opposed to formatted text....
.
Most graphical email clients support HTML email, and many default to it. Many of these clients include both a GUI
Gui
Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...
editor for composing HTML emails and a rendering engine for displaying received HTML emails.
HTML mail allows the sender to properly express quotations
Block quote
A block quotation is a quotation in a written document, that set off from the main text as a paragraph, or block of text, and typically distinguished visually using indentation and a different typeface or smaller size quotation...
(as in inline replying), headings, bulleted lists
Bullet (typography)
In typography, a bullet is a typographical symbol or glyph used to introduce items in a list. For example:*Item 1*Item 2*Item 3...
, emphasized text
Emphasis (typography)
In typography, emphasis is the exaggeration of words in a text with a font in a different style from the rest of the text—to emphasize them.- Methods and use :...
, subscripts and superscripts, and other visual and typographic cues to improve the readability and aesthetics of the message, as well as semantic information encoded within the message, such as the original author and Message-ID of a quote. Long URL
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
s 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 table
Table (information)
A table is a means of arranging data in rows and columns.Production % of goalNorth 4087102%South 4093110% The use of tables is pervasive throughout all communication, research and data analysis. Tables appear in print media, handwritten notes, computer software, architectural...
s, as well as diagrams or mathematical formulae as images, which are otherwise difficult to convey (typically using ASCII art
ASCII art
ASCII art is a graphic design technique that uses computers for presentation and consists of pictures pieced together from the 95 printable characters defined by the ASCII Standard from 1963 and ASCII compliant character sets with proprietary extended characters...
).
Adoption
Since its conception, a number of people have vocally opposed all HTML email (and even MIMEMIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
itself), for a variety of reasons.. The ASCII Ribbon Campaign
ASCII Ribbon Campaign
The ASCII ribbon campaign was an Internet phenomenon started in 1998 advocating that e-mail be sent only in plain text, because of inefficiencies or dangers of using HTML e-mail...
is an internet phenomenon advocating that email should continue to be sent in Human Readable ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
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.
Compatibility
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
Gmail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
.
In particular, the
Separation of style and content
Web style sheets are a form of separation of presentation and content for web design in which the markup of a webpage contains the page's semantic content and structure, but does not define its visual layout . Instead, the style is defined in an external stylesheet file using a style sheet...
point of view. Although workarounds have been developed, this has caused no shortage of frustration among newsletter developers, spawning the grassroots
Grassroots
A grassroots movement is one driven by the politics of a community. The term implies that the creation of the movement and the group supporting it are natural and spontaneous, highlighting the differences between this and a movement that is orchestrated by traditional power structures...
Email Standards Project, which grades email clients on their rendering of an acid test, inspired by those of the Web Standards Project
Web Standards Project
The Web Standards Project is a group of professional web developers dedicated to disseminating and encouraging the use of the web standards recommended by the World Wide Web Consortium, along with other groups and standards bodies....
, and lobbies developers to improve their products. To persuade Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...
to improve rendering in Gmail
Gmail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
, for instance, they published a video montage of grimacing web developers, resulting in attention from an employee.
Style
Some senders may excessively rely upon large, colorful, or distracting fontFont
In typography, a font is traditionally defined as a quantity of sorts composing a complete character set of a single size and style of a particular typeface...
s, making messages more difficult to read. For those especially bothered by this formatting, some user agent
User agent
In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
s make it possible for the reader to partially override the formatting (for instance, Mozilla Thunderbird
Mozilla Thunderbird
Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser...
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 clients, using theContent-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, an equation may lose a superscript and take on an entirely new meaning.)Many mailing list
Electronic mailing list
An electronic mailing list is a special usage of email that allows for widespread distribution of information to many Internet users. It is similar to a traditional mailing list — a list of names and addresses — as might be kept by an organization for sending publications to...
s deliberately block HTML email, either stripping out the HTML part to just leave the plain text part or rejecting the entire message.
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.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
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s), on a modern connection the difference is negligible for most people, especially when compared to images, music files, or other common attachments.
Security vulnerabilities
HTML allows for a link to have a different target than the link's text. This can be used in phishingPhishing
Phishing is a way of attempting to acquire information such as usernames, passwords, and credit card details by masquerading as a trustworthy entity in an electronic communication. Communications purporting to be from popular social web sites, auction sites, online payment processors or IT...
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 bug
Web bug
A web bug is an object that is embedded in a web page or e-mail and is usually invisible to the user but allows checking that a user has viewed the page or e-mail. One common use is in e-mail tracking. Alternative names are web beacon, tracking bug, and tag or page tag...
s (inline content from an external server, such as a picture
Digital image
A digital image is a numeric representation of a two-dimensional image. Depending on whether or not the image resolution is fixed, it may be of vector or raster type...
), 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).
Most email spam is sent in HTML for these reasons, so spam filters sometimes give higher spam scores to HTML messages.
External links
- Dan's Mail Format Site
- Using HTML in E-mail
- HTML Threading: Conventions for use of HTML in email
- Configuring Mail Clients to Send Plain ASCII Text — Argues that HTML (and MIME in general) should never be used in mail
- Configuring Your E-mail Software to Avoid Sending HTML-formatted Messages
- Why HTML in E-Mail is a Bad Idea (written in the late 90s)
- Rebuttal
- HTML Email: The Poll — January 2006 poll and comments on whether the original document is still relevant
- HTML e-mail is STILL evil!!!
- HTML Email Isn't Rich (2003)
- Do we really want to send web pages in e-mail?
- HTML in Email — Ask E.T. Forum (2003)
- A Guide to CSS Support in Email (2006, updated in 2007)
- Andy Budd's blog discussion regarding CSS in MUAs
- The ASCII Ribbon Campaign homepage
- Articles and examples of HTML Email