How to Add Alt Text to Images?

Why Alt Text Matters

Adding accessible and SEO-friendly alt text to images is crucial for modern web content. alt attributes improve visibility in search results, improve usability with screen readers, and ensure HTML documents comply with accessibility standards such as WCAG. Using Aspose.HTML for .NET, you can automate this process and ensure that every image in your HTML documents contains the correct alt content.

Adding alt text is essential for:

Accessibility – Lack of alt text is one of the most common accessibility issues. Screen readers use alt descriptions for visually impaired users.
SEO – Search engines index images more effectively with alt attributes.
Usability – If an image fails to load, alt text provides a meaningful alternative.

In this article, you will learn how to automatically detect missing alt text in an HTML file and add meaningful image descriptions using C#. Before you begin, make sure you have Aspose.HTML for .NET installed. You can add it from NuGet:

1Install-Package Aspose.HTML

Check and Add Alt Text Using C#

Aspose.HTML for .NET offers a robust DOM API that enables easy loading of HTML files, inspection of image elements, modification of their attributes, and saving of the updated document. With these features, you can quickly build automated tools to detect missing alt text and improve accessibility.

Load an HTML document using the HTMLDocument class. It parses the HTML file and gives you full DOM access.
Use the GetElementsByTagName(“img”) method to find all <img> tags.
For each image, read the alt attribute. Use GetAttribute(“alt”) to retrieve the current value.
Check if the alt attribute is missing or empty.
Generate a descriptive alt text. The example uses the image file name, but you can use any logic you prefer.
Use the SetAttribute(“alt”, autoAlt) method to write the new alt text and update the DOM.
Call the Save() method to save the updated HTML to a file.

The following example loads an HTML document, checks every <img> element, and adds descriptive alt text if it is missing.

 1using Aspose.Html;
 2using Aspose.Html.Dom;
 3using Aspose.Html.Dom.Html;
 4using System;
 5using System.IO;
 6...
 7
 8    string inputPath = Path.Combine(DataDir, "document.html");
 9    string outputPath = Path.Combine(DataDir, "document-updated.html");
10
11    // Load the HTML document
12    HTMLDocument document = new HTMLDocument(inputPath);
13
14    // Get all <img> elements in the document
15    HTMLCollection images = document.GetElementsByTagName("img");
16
17    // Iterate through all image elements
18    foreach (Element node in images)
19    {
20        // Ensure the node is an HTMLImageElement
21        HTMLImageElement img = node as HTMLImageElement;
22        if (img != null)
23        {
24            // Read the existing alt attribute
25            string alt = img.GetAttribute("alt");
26
27            // Check if the alt text is missing or empty
28            if (string.IsNullOrWhiteSpace(alt))
29            {
30                // Generate simple descriptive alt text using the image filename
31                string autoAlt = "Image: " + Path.GetFileName(img.Src);
32
33                // Add the generated alt text to the image element
34                img.SetAttribute("alt", autoAlt);
35
36                Console.WriteLine("Alt text added for: " + img.Src + " → \"" + autoAlt + "\"");
37            }
38        }
39    }
40
41    // Save the updated HTML document
42    document.Save(outputPath);

FAQ

1. What happens if an image already has alt text?
The code keeps the existing alt text unchanged. Alt text will only be added to those <img> elements where it was missing.

2. How to programmatically generate meaningful alt text for images?
Programming languages such as C# or Python can be used to parse <img> tags and generate alternative text for them. Alternative text can be created based on the image filename, metadata, or descriptions generated by AI. Always review the output to ensure accuracy and relevance.

3. Should I use automatically generated alt text without modification?
Not always. Simple file names are fine for many images, but for important content, it’s best to manually review and adjust the alt text for clarity and accessibility.

4. Will adding alt text affect how my HTML looks?
No. The alt attribute does not affect the visual display of an image. It is only used by screen readers or displayed when the image does not load.

5. Does Aspose.HTML for .NET support saving to formats other than HTML?
Yes. You can convert HTML to PDF, XPS, images, Markdown, MHTML, and other formats – alt text will be preserved in the output.

6. How long should alt text be?
Ideally, 5-15 words. It should convey the image’s purpose without being overwhelming.

7. Are there any rules for alternative text?
Yes. Alternative text should be concise, descriptive, and clearly reflect the image’s purpose. Avoid unnecessary words like “image”. For purely decorative images, use an empty alt attribute (alt="") so screen readers skip them. For SEO, include relevant keywords naturally without stuffing. Alt text helps search engines understand the content of images and improves indexing.

Conclusion

Adding alt text to images is a crucial step in ensuring that HTML content is both accessible and optimized for search engines. With Aspose.HTML for .NET, you can automate this process, detect missing alt attributes, add meaningful descriptions, and ensure consistent document quality.

Following best practices, ensure alt text is concise, descriptive, and accurately reflects the image’s content or function. Avoid boilerplate placeholders, use alt="" for purely decorative images, and review automatically generated descriptions for clarity and relevance. Applying these principles will ensure your content remains accessible, user-friendly, and SEO-friendly.

See Also

How to Improve Website Accessibility – This article describes how to make your website accessible to users and check the website for compliance with WCAG guidelines using Aspose.HTML for .NET.
Screen Reader Accessibility – Check alt text in C# – You will learn how to design your website for screen reader accessibility and how to check alternative text for screen readers against WCAG guidelines using Aspose.HTML for .NET API.
Free Online Alt Text Checker – This app checks text alternatives for non-text elements with WCAG compliance.

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