Alt text, or alternative text, is an attribute associated with a graphic element on a web page that provides a textual description of the image’s content. In its most common form it appears in HTML as the alt attribute of the image tag, but similar concepts exist for other media elements. Its main purpose is to ensure accessibility: when an image cannot be loaded or displayed, the alternative text takes its place, and screen readers read it aloud for blind or low-vision users.
In SEO, alternative text is considered a relevant optimization factor because it gives search engines information about the content of an image that would otherwise be hard to interpret. An accurate and relevant description can improve a page’s visibility in image searches and contribute to the overall understanding of the topic covered. However, alt text should not be seen as a simple space to insert keywords: artificial use can be counterproductive and penalize the page’s perceived quality.
Where alternative text comes from and what it is for
Alternative text originated as an accessibility tool within the specifications of the hypertext markup language. The basic idea is simple: every non-text content must have an equivalent text alternative, so that no information is lost for those who cannot perceive the visual element. International web content accessibility standards recommend its systematic use for all informative images.
Over the years the concept has extended beyond the single attribute of the image tag to include descriptions for charts, maps, icons, and interactive content. The logic remains unchanged: provide a text equivalent that conveys the same meaning as the original element.
The role of alt text in signals for search engines
Search engines do not “see” images as a person does: they analyze the context, the file name, and especially the associated textual information. Alternative text is therefore one of the few explicit signals an author can provide to indicate the topic of an image. A clear description helps the engine connect the image to the page’s theme and return it in relevant search results.
Good alternative text also contributes to the overall user experience. If an image fails to load, the visitor still reads a sensible description instead of a blank space. This reduces page abandonment and improves the perception of the site’s reliability.
When keywords are relevant
Inserting keywords in alternative text is possible, but it must be done with measure and only when they are truly relevant to the image. Text that faithfully describes the content and naturally includes a relevant term is useful for both accessibility and SEO. Conversely, piling up terms unrelated to the image produces a manipulation effect that search engines tend to ignore or evaluate negatively.
Describe the function of the image, not the details
The fundamental rule is to describe the function of the image in the context of the page, not to list every visual detail. If the image is decorative and adds no information, the alternative text should be empty, so that screen readers ignore it. If the image conveys content, the description must be concise but complete.
Effective alternative text is short, specific, and free of redundant expressions like “image of” or “photo showing”, which are already implicit in the nature of the element. Better to focus on what the image communicates and why it is present on that page. The ideal length varies with the complexity of the content: a few words for an icon, one or two sentences for a detailed photograph.
The most common alt text mistakes
Common mistakes include using generic text like “image1”, mechanically repeating keywords, and completely omitting the attribute. Another mistake is describing details irrelevant to the context, such as background colors, while neglecting the main information. Leaving the same alternative text on different images on the same page is also a practice to avoid.
Comparing different description methods
There are several ways to associate text with a graphic element. The following table compares the main ones, highlighting their uses and limits.
| Type | Where it applies | Main purpose | SEO relevance |
|---|---|---|---|
| Short alternative text | Informative images | Describe the essential content | High |
| Empty alternative text | Decorative images | Exclude the element from reading | None |
| Long description | Complex charts and maps | Explain detailed data | Medium |
| Visible caption | Photographs with context | Add additional information | Medium |
| File name | Any image | Secondary clue about content | Low |
An orderly method for applying alt text
To correctly apply alternative text to a site, it is best to follow an orderly method that considers both accessibility and ranking goals.
- Examine each image and determine whether it is informative or purely decorative.
- For decorative images, leave the alternative text empty.
- For informative images, succinctly describe what they show.
- Check that the description is relevant to the page context.
- Insert any keywords only if consistent with the actual content.
- Avoid repetitions and generic formulas with no informational value.
- Check the result with a screen reader or a verification tool.
- Periodically update descriptions when images change.
Extensions and applications for verification
There are several applications and extensions that allow you to check the presence and quality of alternative texts on a page. These tools flag images without descriptions and help identify those with overly generic texts. Manual checking is still advisable, because only a person can assess whether the description truly conveys the meaning of the image.
Frequently asked questions about alternative text
What exactly is alternative text?
It is a textual description associated with an image or other non-text element, which summarizes its content and allows understanding even for those who cannot see it.
Is alternative text only for accessibility?
No, besides ensuring accessibility it represents a useful signal for search engines, which use it to understand the topic of the image and the page.
How long should alternative text be?
There is no fixed length: it should be short but sufficient to describe what the image communicates, avoiding both excessive detail and overly vague descriptions.
Can I insert keywords in alternative text?
Yes, but only if they are relevant to the image and inserted naturally. The artificial accumulation of terms is discouraged and can be harmful.
What should I write for a decorative image?
For purely decorative images, it is appropriate to leave the alternative text empty, so that screen readers do not announce them and do not disturb the reading.
Does alternative text affect ranking?
It contributes to search engines’ understanding of the page and can favor visibility in image searches, but by itself it does not determine overall ranking.
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