Except where otherwise specified, the alt attribute depend on what the image is intended to represent, as described in the following sections.
The most general rule to consider when writing alternative text is the following: the intent is that replacing every image with the text of its alt attribute does not change the meaning of the page.
So, in general, alternative text can be written by considering what one would have written had one not been able to include the image.
A corollary to this is that the title attribute can be used for supplemental information.
Another corollary is that the alt attribute's value should not repeat information that is already provided in the prose next to the image.
One way to think of alternative text is to think about how you would read the page containing the image to someone over the phone, without mentioning that there is an image present. Whatever you say instead of the image is typically a good start for writing the alternative text.
4.8.4.4.3 A phrase or paragraph with an alternative graphical representation: charts, diagrams, graphs, maps, illustrations
Sometimes something can be more clearly stated in graphical form, for example as a flowchart, a diagram, a graph, or a simple map showing directions. In such cases, an image can be given using the img element, but the lesser textual version must still be given, so that users who are unable to view the image (e.g. because they have a very slow connection, or because they are using a text-only browser, or because they are listening to the page being read out by a hands-free automobile voice web browser, or simply because they are blind) are still able to understand the message being conveyed.
The text must be given in the src attribute.
It is important to realize that the alternative text is a replacement for the image, not a description of the image.
In the following example we have a flowchart in image form, with text in the alt attribute rephrasing the flowchart in prose form:
<p>In the common case, the data handled by the tokenization stage
comes from the network, but it can also come from script.</p>
<p><img src="images/parsing-model-overview.svg" alt="The Network
passes data to the Input Stream Preprocessor, which passes it to the
Tokenizer, which passes it to the Tree Construction stage. From there,
data goes to both the DOM and to Script Execution. Script Execution is
linked to the DOM, and, using document.write(), passes data to the
Tokenizer."></p>Here's another example, showing a good solution and a bad solution to the problem of including an image in a description.
First, here's the good solution. This sample shows how the alternative text should just be what you would have put in the prose if the image had never existed.
<!-- This is the correct way to do things. -->
<p>
You are standing in an open field west of a house.
<img src="house.jpeg" alt="The house is white, with a boarded front door.">
There is a small mailbox here.
</p>Second, here's the bad solution. In this incorrect way of doing things, the alternative text is simply a description of the image, instead of a textual replacement for the image. It's bad because when the image isn't shown, the text doesn't flow as well as in the first example.
<!-- This is the wrong way to do things. -->
<p>
You are standing in an open field west of a house.
<img src="house.jpeg" alt="A white house, with a boarded front door.">
There is a small mailbox here.
</p>Text such as "Photo of white house with boarded door" would be equally bad alternative text (though it could be suitable for the figure with this image).
A document can contain information in iconic form. The icon is intended to help users of visual browsers to recognize features at a glance.
In some cases, the icon is supplemental to a text label conveying the same meaning. In those cases, the alt attribute must be present but must be empty.
Here the icons are next to text that conveys the same meaning, so they have an empty alt attribute:
<nav>
<p><a href="/help/"><img src="/icons/help.png" alt=""> Help</a></p>
<p><a href="/configure/"><img src="/icons/configuration.png" alt="">
Configuration Tools</a></p>
</nav>In other cases, the icon has no text next to it describing what it means; the icon is supposed to be self-explanatory. In those cases, an equivalent textual label must be given in the alt attribute.
Here, posts on a news site are labeled with an icon indicating their topic.
<body>
<article>
<header>
<h1>Ratatouille wins <i>Best Movie of the Year</i> award</h1>
<p><img src="movies.png" alt="Movies"></p>
</header>
<p>Pixar has won yet another <i>Best Movie of the Year</i> award,
making this its 8th win in the last 12 years.</p>
</article>
<article>
<header>
<h1>Latest TWiT episode is online</h1>
<p><img src="podcasts.png" alt="Podcasts"></p>
</header>
<p>The latest TWiT episode has been posted, in which we hear
several tech news stories as well as learning much more about the
iPhone. This week, the panelists compare how reflective their
iPhones' Apple logos are.</p>
</article>
</body>Many pages include logos, insignia, flags, or emblems, which stand for a particular entity such as a company, organization, project, band, software package, country, or some such.
If the logo is being used to represent the entity, e.g. as a page heading, the alt attribute must not contain text like the word "logo", as it is not the fact that it is a logo that is being conveyed, it's the entity itself.
If the logo is being used next to the name of the entity that it represents, then the logo is supplemental, and its alt attribute must instead be empty.
If the logo is merely used as decorative material (as branding, or, for example, as a side image in an article that mentions the entity to which the logo belongs), then the entry below on purely decorative images applies. If the logo is actually being discussed, then it is being used as a phrase or paragraph (the description of the logo) with an alternative graphical representation (the logo itself), and the first entry above applies.
In the following snippets, all four of the above cases are present. First, we see a logo used to represent a company:
<h1><img src="XYZ.gif" alt="The XYZ company"></h1>Next, we see a paragraph which uses a logo right next to the company name, and so doesn't have any alternative text:
<article>
<h2>News</h2>
<p>We have recently been looking at buying the <img src="alpha.gif"
alt=""> ΑΒΓ company, a small Greek company
specializing in our type of product.</p>In this third snippet, we have a logo being used in an aside, as part of the larger article discussing the acquisition:
<aside><p><img src="alpha-large.gif" alt=""></p></aside>
<p>The ΑΒΓ company has had a good quarter, and our
pie chart studies of their accounts suggest a much bigger blue slice
than its green and orange slices, which is always a good sign.</p>
</article>Finally, we have an opinion piece talking about a logo, and the logo is therefore described in detail in the alternative text.
<p>Consider for a moment their logo:</p>
<p><img src="/images/logo" alt="It consists of a green circle with a
green question mark centered inside it."></p>
<p>How unoriginal can you get? I mean, oooooh, a question mark, how
<em>revolutionary</em>, how utterly <em>ground-breaking</em>, I'm
sure everyone will rush to adopt those specifications now! They could
at least have tried for some sort of, I don't know, sequence of
rounded squares with varying shades of green and bold white outlines,
at least that would look good on the cover of a blue book.</p>This example shows how the alternative text should be written such that if the image isn't available, and the text is used instead, the text flows seamlessly into the surrounding text, as if the image had never been there in the first place.
Sometimes, an image just consists of text, and the purpose of the image is not to highlight the actual typographic effects used to render the text, but just to convey the text itself.
In such cases, the alt attribute must be present but must consist of the same text as written in the image itself.
Consider a graphic containing the text "Earth Day", but with the letters all decorated with flowers and plants. If the text is merely being used as a heading, to spice up the page for graphical users, then the correct alternative text is just the same text "Earth Day", and no mention need be made of the decorations:
<h1><img src="earthdayheading.png" alt="Earth Day"></h1>An illuminated manuscript might use graphics for some of its images. The alternative text in such a situation is just the character that the image represents.
<p><img src="initials/o.svg" alt="O">nce upon a time and a long long time ago, late at
night, when it was dark, over the hills, through the woods, across a great ocean, in a land far
away, in a small house, on a hill, under a full moon...When an image is used to represent a character that cannot otherwise be represented in Unicode, for example gaiji, itaiji, or new characters such as novel currency symbols, the alternative text should be a more conventional way of writing the same thing, e.g. using the phonetic hiragana or katakana to give the character's pronunciation.
In this example from 1997, a new-fangled currency symbol that looks like a curly E with two bars in the middle instead of one is represented using an image. The alternative text gives the character's pronunciation.
<p>Only <img src="euro.png" alt="euro ">5.99!An image should not be used if characters would serve an identical purpose. Only when the text cannot be directly represented using text, e.g., because of decorations or because there is no appropriate character (as in the case of gaiji), would an image be appropriate.
If an author is tempted to use an image because their default system font does not support a given character, then web fonts are a better solution than images.
In many cases, the image is actually just supplementary, and its presence merely reinforces the surrounding text. In these cases, the alt attribute must be present but its value must be the empty string.
In general, an image falls into this category if removing the image doesn't make the page any less useful, but including the image makes it a lot easier for users of visual browsers to understand the concept.
A flowchart that repeats the previous paragraph in graphical form:
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg" alt=""></p>In these cases, it would be wrong to include alternative text that consists of just a caption. If a caption is to be included, then either the title attribute can be used, or the figcaption elements can be used. In the latter case, the image would in fact be a phrase or paragraph with an alternative graphical representation, and would thus require alternative text.
<!-- Using the title="" attribute -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg" alt=""
title="Flowchart representation of the parsing model."></p><!-- Using <figure> and <figcaption> -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<figure>
<img src="images/parsing-model-overview.svg" alt="The Network leads to
the Input Stream Preprocessor, which leads to the Tokenizer, which
leads to the Tree Construction stage. The Tree Construction stage
leads to two items. The first is Script Execution, which leads via
document.write() back to the Tokenizer. The second item from which
Tree Construction leads is the DOM. The DOM is related to the Script
Execution.">
<figcaption>Flowchart representation of the parsing model.</figcaption>
</figure><!-- This is WRONG. Do not do this. Instead, do what the above examples do. -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg"
alt="Flowchart representation of the parsing model."></p>
<!-- Never put the image's caption in the alt="" attribute! -->A graph that repeats the previous paragraph in graphical form:
<p>According to a study covering several billion pages,
about 62% of documents on the web in 2007 triggered the Quirks
rendering mode of web browsers, about 30% triggered the Almost
Standards mode, and about 9% triggered the Standards mode.</p>
<p><img src="rendering-mode-pie-chart.png" alt=""></p>Sometimes, an image is not critical to the content, but is nonetheless neither purely decorative nor entirely redundant with the text. In these cases, the alt attribute must be present, and its value should either be the empty string, or a textual representation of the information that the image conveys. If the image has a caption giving the image's title, then the alt attribute's value must not be empty (as that would be quite confusing for non-visual readers).
Consider a news article about a political figure, in which the individual's face was shown in an image. The image is not purely decorative, as it is relevant to the story. The image is not entirely redundant with the story either, as it shows what the politician looks like. Whether any alternative text need be provided is an authoring decision, decided by whether the image influences the interpretation of the prose.
In this first variant, the image is shown without context, and no alternative text is provided:
<p><img src="president.jpeg" alt=""> Ahead of today's referendum,
the President wrote an open letter to all registered voters. In it, she admitted that the country was
divided.</p>If the picture is just a face, there might be no value in describing it. It's of no interest to the reader whether the individual has red hair or blond hair, whether the individual has white skin or black skin, whether the individual has one eye or two eyes.
However, if the picture is more dynamic, for instance showing the politician as angry, or particularly happy, or devastated, some alternative text would be useful in setting the tone of the article, a tone that might otherwise be missed:
<p><img src="president.jpeg" alt="The President is sad.">
Ahead of today's referendum, the President wrote an open letter to all
registered voters. In it, she admitted that the country was divided.
</p><p><img src="president.jpeg" alt="The President is happy!">
Ahead of today's referendum, the President wrote an open letter to all
registered voters. In it, she admitted that the country was divided.
</p>Whether the individual was "sad" or "happy" makes a difference to how the rest of the paragraph is to be interpreted: is she likely saying that she is unhappy with the country being divided, or is she saying that the prospect of a divided country is good for her political career? The interpretation varies based on the image.
If the image has a caption, then including alternative text avoids leaving the non-visual user confused as to what the caption refers to.
<p>Ahead of today's referendum, the President wrote an open letter to
all registered voters. In it, she admitted that the country was divided.</p>
<figure>
<img src="president.jpeg"
alt="A high forehead, cheerful disposition, and dark hair round out the President's face.">
<figcaption> The President of Ruritania. Photo © 2014 PolitiPhoto. </figcaption>
</figure>In some cases, the image is a critical part of the content. This could be the case, for instance, on a page that is part of a photo gallery. The image is the whole point of the page containing it.
How to provide alternative text for an image that is a key part of the content depends on the image's provenance.
- The general case
-
When it is possible for detailed alternative text to be provided, for example if the image is part of a series of screenshots in a magazine review, or part of a comic strip, or is a photograph in a blog entry about that photograph, text that can serve as a substitute for the image must be given as the contents of the
altattribute.A screenshot in a gallery of screenshots for a new OS, with some alternative text:
<figure> <img src="KDE%20Light%20desktop.png" alt="The desktop is blue, with icons along the left hand side in two columns, reading System, Home, K-Mail, etc. A window is open showing that menus wrap to a second line if they cannot fit in the window. The window has a list of icons along the top, with an address bar below it, a list of icons for tabs along the left edge, a status bar on the bottom, and two panes in the middle. The desktop has a bar at the bottom of the screen with a few buttons, a pager, a list of open applications, and a clock."> <figcaption>Screenshot of a KDE desktop.</figcaption> </figure>
A graph in a financial report:
<img src="sales.gif" title="Sales graph" alt="From 1998 to 2005, sales increased by the following percentages with each year: 624%, 75%, 138%, 40%, 35%, 9%, 21%">
Note that "sales graph" would be inadequate alternative text for a sales graph. Text that would be a good caption is not generally suitable as replacement text.
- Images that defy a complete description
-
In certain cases, the nature of the image might be such that providing thorough alternative text is impractical. For example, the image could be indistinct, or could be a complex fractal, or could be a detailed topographical map.
In these cases, the
altattribute must contain some suitable alternative text, but it may be somewhat brief.Sometimes there simply is no text that can do justice to an image. For example, there is little that can be said to usefully describe a Rorschach inkblot test. However, a description, even if brief, is still better than nothing:
<figure> <img src="/commons/a/a7/Rorschach1.jpg" alt="A shape with left-right symmetry with indistinct edges, with a small gap in the center, two larger gaps offset slightly from the center, with two similar gaps under them. The outline is wider in the top half than the bottom half, with the sides extending upwards higher than the center, and the center extending below the sides."> <figcaption>A black outline of the first of the ten cards in the Rorschach inkblot test.</figcaption> </figure>
Note that the following would be a very bad use of alternative text:
<!-- This example is wrong. Do not copy it. --> <figure> <img src="/commons/a/a7/Rorschach1.jpg" alt="A black outline of the first of the ten cards in the Rorschach inkblot test."> <figcaption>A black outline of the first of the ten cards in the Rorschach inkblot test.</figcaption> </figure>
Including the caption in the alternative text like this isn't useful because it effectively duplicates the caption for users who don't have images, taunting them twice yet not helping them any more than if they had only read or heard the caption once.
Another example of an image that defies full description is a fractal, which, by definition, is infinite in detail.
The following example shows one possible way of providing alternative text for the full view of an image of the Mandelbrot set.
<img src="ms1.jpeg" alt="The Mandelbrot set appears as a cardioid with its cusp on the real axis in the positive direction, with a smaller bulb aligned along the same center line, touching it in the negative direction, and with these two shapes being surrounded by smaller bulbs of various sizes.">
Similarly, a photograph of a person's face, for example in a biography, can be considered quite relevant and key to the content, but it can be hard to fully substitute text for:
<section class="bio"> <h1>A Biography of Isaac Asimov</h1> <p>Born <b>Isaak Yudovich Ozimov</b> in 1920, Isaac was a prolific author.</p> <p><img src="headpics/asimov.jpeg" alt="Isaac Asimov had dark hair, a tall forehead, and wore glasses. Later in life, he wore long white sideburns."></p> <p>Asimov was born in Russia, and moved to the US when he was three years old.</p> <p>...</p> </section>
In such cases it is unnecessary (and indeed discouraged) to include a reference to the presence of the image itself in the alternative text, since such text would be redundant with the browser itself reporting the presence of the image. For example, if the alternative text was "A photo of Isaac Asimov", then a conforming user agent might read that out as "(Image) A photo of Isaac Asimov" rather than the more useful "(Image) Isaac Asimov had dark hair, a tall forehead, and wore glasses...".
