Alt text, short for alternative text, is a short written description attached to an image that stands in for the picture whenever it cannot be seen. On the web it lives in the alt attribute of the HTML img element. A screen reader speaks it to a blind user, a browser shows it when the file fails to load, and search engines, ad servers and AI agents parse it. It exists because images were added to a text medium in 1993, and the attribute kept their information available to text-only browsers and, later, to people who cannot see. For marketers it touches accessibility law, search visibility and, since October 1, 2026, Google's call for human review of machine-written image descriptions.

How the attribute works

The mechanism is one line of markup: an image tag carrying a source address and an alt value. Google's documentation offers "Dalmatian puppy playing fetch" as its best example and warns that a string of stacked keywords may get a site seen as spam. When a page loads, the browser builds an accessibility tree, a structured copy of the page for assistive software, and the alt value becomes the image's accessible name. Screen readers such as JAWS, NVDA and Apple's VoiceOver announce an image, then speak the text or send it to a braille display.

The HTML Standard, maintained by the Web Hypertext Application Technology Working Group (WHATWG), sets the core rule: except in specified cases the attribute must be present and non-empty, and replacing every image with its alt text should not change the meaning of the page. Three states follow:

  • Descriptive value. The text is read in place of the image. For an image that is the only content of a link, the value must describe the destination.
  • Empty value (alt=""). Known as null alt, it marks the image as decorative, so assistive technology skips it, as with spacers or icons beside a text label.
  • Missing attribute. Software has nothing to read and may fall back to announcing the file name.

The Web Content Accessibility Guidelines (WCAG), published by the World Wide Web Consortium (W3C), document the misuse of each state as a named failure: F65 for omitting the attribute, F30 for file names or placeholder text, F39 for values such as "spacer" on images that should be ignored. Context decides the wording: the W3C labels one globe image "International Travel" on a travel site and "International Campuses" on a university site.

Other formats carry the same function: a title element in SVG graphics, labelling attributes from the Accessible Rich Internet Applications (ARIA) specification, an Alt entry in PDF files and an og:image:alt sub-property in the Open Graph protocol behind social link previews.

Advertising raises the question of who writes the text. Publishers trafficking direct-sold image creatives in Google Ad Manager get an optional alt text field, which Google says assists visually impaired users and shows when images are disabled. Programmatic pipes carry no equivalent. The IAB Tech Lab's Advertising Common Object Model (AdCOM) 1.0, the object layer beneath OpenRTB 3.0, defines a banner by image URL and link, and a native image asset by URL, width, height and type, with no description field. For HTML5 and tag-based creatives bought through a demand-side platform (DSP), the alt text is whatever the creative developer wrote into the markup. Social ad tools vary: a 2023 Social Media Examiner guide found no custom field in Meta Ads Manager or Twitter Ads Manager, while LinkedIn Campaign Manager took up to 300 characters.

From text browsers to accessibility law

Marc Andreessen, then building the Mosaic browser at the National Center for Supercomputing Applications (NCSA), proposed the img tag on the www-talk mailing list on February 25, 1993. A June 1993 Internet Engineering Task Force (IETF) draft of HTML by Tim Berners-Lee and Dan Connolly already listed ALT beside SRC and ALIGN. RFC 1866, the HTML 2.0 specification of November 1995, made the attribute official but optional.

Accessibility turned the fallback into an obligation. WCAG 1.0, published on May 5, 1999, made text equivalents for images its first checkpoint, and HTML 4.01 made the attribute mandatory on img and area elements that December. WCAG 2.0, a W3C Recommendation from December 11, 2008, recast the rule as success criterion 1.1.1, Non-text Content, at Level A, the lowest conformance tier, and WCAG 2.2 kept it unchanged on October 5, 2023.

Law then borrowed the standard. The European Accessibility Act, Directive (EU) 2019/882, adopted on April 17, 2019, has applied since June 28, 2025 to e-commerce, banking and other consumer services. Compliance work generally relies on EN 301 549, a European standard incorporating WCAG 2.1 Level AA, though law firm Osborne Clarke expected website requirements to track WCAG 2.2. In the US, the Department of Justice adopted WCAG 2.1 AA for state and local government sites in 2024 and, on April 20, 2026, delayed compliance by a year. Private companies face litigation instead: plaintiffs filed 3,117 website accessibility lawsuits in federal court in 2025, up 27% on 2024, according to law firm Seyfarth Shaw.

Automation arrived in parallel. Facebook began generating automatic alternative text on April 4, 2016, recognising around 100 concepts at launch, according to its engineering blog. Mozilla's Firefox 130, released in 2024, added an on-device model that drafts alt text for images in PDF files and asks the user to confirm it.

Why marketers deal with it

Search is the commercial driver. Google's image guidance, last updated on March 2, 2026, calls alt text the most important attribute for image metadata and says Google reads it alongside computer vision and page content. Google's May 2026 guide to generative AI features told sites to add descriptive alt text to every image, and LinkedIn's organic growth team listed specific alt text in its February 2026 optimisation guide. Pinterest extracts alt text alongside titles, board names and schema.org data when assigning interests to pins.

Agents widen the readership. Google named three ways agents perceive a site: screenshots, HTML source and the accessibility tree, the same structure screen readers use. SEO practitioner Lily Ray stressed alt text and file names so that meaning carried by design reaches "bots and agents as well".

The gap remains wide. WebAIM's March 2026 analysis of the top one million home pages found missing alt text on 53.1% of them, down from 68% in 2019. Some 16.2% of images lacked it, about 10.8 per page, and 45% of those were linked images, producing links that announce nothing. Ad creative sits at the margin: Google's Demand Gen guidance treats alt text as an accessibility consideration outside the formal specifications.

Where it falls short

The two audiences pull apart. Text written for a ranking system drifts towards keywords; text written for a listener describes purpose.

Length is disputed. A widely repeated rule caps alt text at 125 characters on the claim that screen readers truncate beyond it, yet no W3C document sets a limit. Tests cited in a W3C discussion found an older JAWS version split long text into 125-character chunks, and accessibility specialist Eric Eggert called the cap a myth in February 2022. Participants there noted the real constraint: a listener cannot pause midway through an attribute.

Automation misreads. Leonie Watson, chair of the W3C Board of Directors, described ChatGPT insisting a telephone on her shelf was beige at FFConf on November 8, 2024, and said she would not base "any kind of big decisions" on such output. Overlay widgets promising automatic fixes drew sanctions: the Federal Trade Commission (FTC) approved a $1 million final order against accessiBe on April 22, 2025 over claims its tool could make any site WCAG-compliant. Joost de Valk's May 2026 web specification lists overlays under "avoid".

Alt attributes are also among the hiding places for prompt injection aimed at AI agents, since text a sighted visitor never sees is still read by a model.

Not the same as

Title attribute. The HTML Standard reserves title for supplementary information, often a tooltip: it adds to an image, while alt replaces it.

Caption. A figcaption is visible to everyone. Alt text that copies it is heard twice.

Long description. Charts need more than a phrase, so WCAG pairs short alt text with a longer description linked through aria-describedby or placed nearby; the old longdesc attribute is obsolete.

Metadata and OCR. Structured data and embedded IPTC fields describe images for machines; an IPTC alt text field added in 2021 reaches a screen reader only if a content system copies it into the page. Optical character recognitionextracts text inside an image automatically; alt text comes from an author. "Alt tag" is a misnomer: alt is an attribute.

Recent developments

On October 1, 2026, Google rewrote the accuracy section of its guidance on generative AI content, calling a manual check critical and extending it to alternate text attached to images, because such text can surface in search results. The page sets no sampling threshold and requires no record of review.

Tooling pulls towards generation at scale. WordPress.com's agent connections can find images missing alt text across a media library and suggest descriptions. Platforms keep adding fields: Threads introduced alt text on August 21, 2024.

Regulation raises the floor. The European Accessibility Act has applied to ebooks and the services selling them since June 28, 2025, with genuine alternative text part of compliance, while UK government testing between 2022 and 2024 found accessibility problems on nearly all of more than 1,200 public sector sites. Whether descriptions generated as fast as the images can meet those standards without the human pass Google now requests is unresolved.

Timeline

  • February 25, 1993: Marc Andreessen proposes the img tag on the www-talk mailing list
  • June 1993: An IETF draft of HTML by Tim Berners-Lee and Dan Connolly lists ALT as an img attribute
  • November 1995: RFC 1866, the HTML 2.0 specification, formalises ALT as an optional attribute
  • May 5, 1999: WCAG 1.0 makes text equivalents for images its first checkpoint
  • December 1999: HTML 4.01 makes alt mandatory on img and area elements
  • December 11, 2008: WCAG 2.0 introduces success criterion 1.1.1, Non-text Content, at Level A
  • April 4, 2016: Facebook launches automatic alternative text, recognising around 100 concepts
  • April 17, 2019: The European Accessibility Act, Directive (EU) 2019/882, is adopted
  • November 2021: IPTC Photo Metadata Standard version 2021.1 adds an accessibility alt text field
  • February 2022: Eric Eggert publishes his rebuttal of the 125-character alt text limit
  • October 5, 2023: WCAG 2.2 is published, retaining success criterion 1.1.1
  • April 2024: The US Department of Justice adopts WCAG 2.1 AA for state and local government web content
  • 2024: Firefox 130 adds on-device alt text generation for images in PDF files
  • August 21, 2024: Threads adds alt text support
  • November 8, 2024: Leonie Watson discusses AI image descriptions and overlays at FFConf
  • January 3, 2025: The FTC announces a proposed $1 million order against accessiBe
  • April 22, 2025: The FTC approves the final accessiBe order
  • June 28, 2025: European Accessibility Act requirements begin to apply
  • March 2, 2026: Google updates its image SEO best practices documentation
  • March 2026: WebAIM reports missing alt text on 53.1% of the top one million home pages
  • April 20, 2026: The Department of Justice extends its Title II compliance dates by one year
  • May 15, 2026: Google publishes its guide to optimising for generative AI features in Search
  • October 1, 2026: Google extends its manual review expectation for AI content to image alternate text
  • April 26, 2027: First Title II web accessibility compliance date for large US public entities
  • April 26, 2028: Title II compliance date for smaller public entities and special district governments

Summary

Who. Content authors, SEO teams, creative developers and publisher ad operations staff write alt text; the WHATWG and the W3C define it; screen reader users, search engines, ad servers and AI agents read it; the European Union through the European Accessibility Act, the US Department of Justice and the FTC set or enforce the rules around it.

What. A short text alternative for an image, usually the value of the HTML alt attribute, that replaces the picture when it cannot be seen. A descriptive value is read aloud, an empty value marks an image as decorative, and a missing value is a WCAG failure under success criterion 1.1.1.

When. The attribute dates from 1993 drafts of HTML, became official in HTML 2.0 in November 1995, mandatory in HTML 4.01 in December 1999 and a Level A accessibility requirement in WCAG 2.0 in December 2008. The European Accessibility Act has applied since June 28, 2025, and Google extended its human review expectation to AI-written alt text on October 1, 2026.

Where. In web pages, email, PDF files, social posts and ad creatives, wherever an image appears, and in the accessibility tree browsers build for assistive technology and, increasingly, for AI agents.

Why. Images carry information that some people and most software cannot see. Alt text puts that information into words, serving accessibility law, image search and machine readers at once, and it puts pressure on organisations that now generate both pictures and their descriptions automatically.