Optimising Alt Tags for Images: Importance for Accessibility and SEO

Introduction to optimising Alt tags

Alt tags are an often overlooked aspect of web search engine optimisation (SEO). However, they play a crucial role both for your site’s accessibility and for its optimisation for search engines (SEO). An Alt tag, or alternative text, is a brief description of an image, which is displayed when the image cannot be loaded on a web page. It is also used by assistive technologies, such as screen readers, to help visually impaired users understand the content of an image.

The importance of Alt tags for accessibility

In terms of accessibility, Alt tags are essential. They allow people who use screen readers and other assistive technologies to understand the content of your website. Without Alt tags, these users could miss important information and have a poorer user experience. Moreover, even those who do not use assistive technologies can benefit from Alt tags. For example, if an image does not load properly, the Alt tag can provide useful context.

The role of Alt tags in SEO

In addition to their vital role in accessibility, Alt tags also play an important role in search engine optimisation. Search engines, such as Google, cannot “see” images in the same way humans do, and therefore rely on Alt tags to understand their content. Well-optimised Alt text can help your site rank higher for relevant keywords and appear in image search results. Thus, not only do Alt tags improve your site’s accessibility, but they can also lead to increased organic search traffic.

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Legacy of accessibility standards and the role of the attribute alt

When Tim Berners-Lee published the first proposal for the World Wide Web in 1989, he implicitly introduced the notion of a universal internet where content would be readable by any machine. Very early on, the W3C understood that part of the population cannot absorb visual information and included in the HTML 2.0 (1995) specification the attribute alt. From there, regulatory bodies such as the US Section 508, the European Web Accessibility Directive and then the General Framework for Improving Accessibility (RGAA) in France, took up the idea and turned it into a legal obligation. Ignoring or incorrectly completing an alt tag therefore exposes you to fines or litigation, just like a shop without an access ramp.

Netflix’s experience in 2016 illustrates the importance of these standards. After a class action brought by the National Association of the Deaf, the platform had to overhaul its multimedia ecosystem to include subtitles and textual descriptions. Even though Netflix mostly deals with video, the case boosted the entire sector: the number of companies carrying out WCAG compliance audits doubled the following year, and with them the attention paid to attributes alt. In other words, the legislator has made “accessible SEO” an industry standard.

Direct impact on the user experience: from speech synthesis to offline browsing

On a well-designed site, the journey of a blind user differs little from that of a sighted user: they simply listen to the descriptions. A screen reader such as JAWS translates the HTML structure into audio signals; the synthetic voice jumps from heading to heading, reads links and, when it reaches an image, automatically switches to the attribute alt. If it is empty or irrelevant (“image1.jpg ), the experience becomes a labyrinth. User tests carried out by the University of Lyon show that 72 % of blind participants leave a page if they encounter more than two images without a textual alternative.

But the issue is not limited to disability. When 3G connections struggle, the browser may not display the image and instead show the text alt. In the archives of the New York Times website, viewing in aeroplane mode brings up descriptions such as “Front-page photograph of the Apollo 11 launch, July 16 1969 . The user understands the historical context despite the absence of visuals. The same scenario occurs in countries where data costs are high; a simple tag alt becomes the main channel of visual information.

Correlation between visual description and semantic relevance

Since the Hummingbird algorithm (2013) and especially RankBrain (2015), Google has enhanced a page’s relevance using semantic signals. The attribute alt then plays a role equivalent to internal link anchors: it provides lexical context. If your product page mentions “waterproof Gore-Tex® hiking shoes” and the image displays a alt generic label like “shoes”, the algorithm loses an essential clue and the long-tail query “women’s goretex trekking boots” may slip through your fingers.

Conversely, Patagonia experimented in 2020 with rewording 18,000 tags alt using key phrases taken from their internal search tool. Result: +12 % organic traffic on Google Images and +5.8 % e-commerce conversions. Even more interesting: CTR on the classic universal search increased by 2 points, proof that a clear description of images also strengthens the overall semantic coherence of the HTML document.

Technical SEO: how tags alt influence indexing and ranking

Googlebot behaves like a browser without a graphical interface. It reads the markup, follows links, assesses the structure and records the text. Images, however, are handled by a separate processor that attempts to recognise shapes via computer vision. To reduce this costly step, Google uses tags alt as a shortcut. A Searchmetrics study (2022) reveals that 30 % of thumbnails displayed in the “Associated images” section of a SERP come from pages where the main keyword appears in the attribute alt.

The influence does not stop at the “Images” vertical. On the main SERP, the “Top Stories” block and rich results (recipes, products, how-to) include thumbnails. These thumbnails are weighted by the semantic relevance of the alt combined with the