Definition
Alt text (alternative text) is an HTML attribute applied to image elements that provides a text description of the image’s content and function. This attribute serves multiple critical purposes: making visual content accessible to users with visual impairments who use screen readers, providing context when images cannot load, and helping search engines understand and properly index image content. Alt text is embedded directly in the HTML code using the alt attribute within an image tag.
Unlike image captions or surrounding text that might describe an image, alt text is specifically designed to replace the image when it cannot be seen or displayed. Properly implemented alt text conveys both the content and the function of images, ensuring that no information is lost when visual elements are inaccessible. This creates an equivalent experience for all users regardless of visual ability or technical limitations.
Key characteristics of Alt Text include:
- HTML implementation using the alt attribute within img tags:
<img src="image.jpg" alt="Descriptive text here"> - Primary role in making websites accessible to visually impaired users
- Secondary function as an SEO element that helps images appear in search results
- Contextual descriptions that convey both what an image shows and why it matters
- Varying length requirements based on image complexity and importance
- Different implementation approaches for decorative versus informational images
- Text that displays in place of images when they fail to load
- Required element for compliance with accessibility standards including WCAG
- Different needs for functional images like buttons versus content images
- Balance between descriptiveness and conciseness
History of Alt Text
Alt text has evolved alongside web accessibility standards and search technology:
1995: HTML 2.0 specification introduces the alt attribute for images, establishing the foundation for image accessibility on the web.
1997-1999: Early screen readers begin supporting alt text, though implementation and usage remain inconsistent.
1999-2000: Web Content Accessibility Guidelines (WCAG) 1.0 establishes alt text as a priority checkpoint for accessibility compliance.
2001-2005: As search engines develop image search capabilities, alt text begins serving a dual role for both accessibility and SEO.
2006-2008: The rise of image-heavy web design increases awareness of alt text importance, though implementation quality varies widely.
2009-2012: WCAG 2.0 refines alt text guidelines, providing more specific recommendations for different image types.
2013-2016: Mobile growth and variable connection speeds highlight the user experience benefits of alt text when images load slowly or fail to display.
2017-2020: Machine learning advancements improve automated image recognition, but alt text remains superior for conveying context and meaning.
2021-2025: Increased legal enforcement of accessibility standards elevates alt text from best practice to essential requirement for many websites.
Types of Alt Text Implementations
Alt text varies based on image purpose and context:
Descriptive Alt Text: Detailed descriptions for informational images that convey both visual content and its significance (e.g., “Bar graph showing sales increase of 25% in Q4 2024 compared to previous year”).
Functional Alt Text: Descriptions that focus on an image’s purpose rather than appearance, particularly for images serving as buttons or controls (e.g., “Search button” or “Download PDF”).
Decorative Image Handling: Empty alt attributes (alt=””) for purely decorative images that add no information, allowing screen readers to skip them entirely.
Product Image Alt Text: Specific format for e-commerce including product name, model, and key features (e.g., “Apple iPhone 13 Pro in Sierra Blue with triple camera system”).
Chart and Infographic Alt Text: Comprehensive descriptions of data visualizations that capture key trends and conclusions rather than just identifying the image type.
Logo Alt Text: Brand identification that typically includes the company name and sometimes tagline (e.g., “Nike logo – Just Do It”).
Image Link Alt Text: Descriptions that indicate both what the image shows and where the link leads (e.g., “Product catalog – Click to browse all items”).
Gallery Image Alt Text: Contextual descriptions for images within collections, often with sequential relationships to other images.
Empty Alt Text: Null alt attribute for images that should be ignored by assistive technology because they are decorative or redundant.
Long Descriptions: Extended implementations for complex images where brief alt text is insufficient, sometimes using aria-describedby to link to longer descriptions.
Importance in Modern SEO
Alt text maintains significant relevance in contemporary SEO practice for several compelling reasons:
As a direct ranking factor for image search, alt text provides essential context that helps search engines understand and categorize visual content. With image search driving substantial traffic for many industries—particularly retail, travel, and education—properly optimized alt text can significantly expand a website’s search visibility beyond traditional text search. This creates additional entry points to websites and supports broader content discovery.
Beyond image search visibility, alt text contributes to topical relevance signals for the page as a whole. By reinforcing page themes and keywords through descriptive image text, alt attributes help search engines build a more complete understanding of page content and subject matter expertise. This comprehensive content analysis can positively influence rankings for related queries.
The strong correlation between thoroughness in technical SEO implementation and overall site quality makes alt text an important quality signal. Sites that consistently implement proper alt text demonstrate attention to detail and technical best practices, potentially influencing how search engines evaluate overall site quality and trustworthiness.
From a user experience perspective, alt text ensures content remains accessible when images fail to load due to slow connections or technical issues. This resilience aligns with Google’s increasing emphasis on page experience as a ranking factor, particularly for mobile users who may experience variable connection quality.
For websites in industries with substantial ADA (Americans with Disabilities Act) compliance requirements, such as government, education, healthcare, and financial services, proper alt text implementation helps avoid legal issues that could damage brand reputation and indirectly impact SEO performance through negative publicity or reduced site credibility.
The growth of voice search and digital assistants has created new importance for descriptive text elements that can be processed and returned in audio format. Well-crafted alt text helps these systems understand and potentially reference image content in response to voice queries.
As machine learning plays an increasing role in search algorithms, the alignment between human-written alt text and computer vision analysis of images creates reinforcing signals about image content. When manually created alt text confirms what image recognition algorithms detect, this consistency may strengthen relevance signals.
For e-commerce particularly, product image alt text directly impacts conversion potential by making product details accessible to all users and improving findability in product image searches. The direct business impact of this visibility makes alt text optimization a high-ROI activity in retail SEO strategies.