Find Images Missing Alt Text in Your HTML

Alt text is the short written description attached to an image with the HTML alt attribute. It is read aloud by screen readers, shown when an image fails to load, and used by search engines to understand what a picture depicts. Despite being one of the simplest accessibility wins available, missing and low-quality alt text remains one of the most common issues on the web — and an alt text checker is the fastest way to find and fix it.

This guide explains what alt text is, why it matters for both accessibility and SEO, how to write it well, and how to use a checker to audit your pages in seconds.

What Is Alt Text?

Alt text (short for "alternative text") lives inside the image tag, like this: <img src="chart.png" alt="Bar chart of 2025 revenue by region">. It is the text alternative for an image — what a person should understand if they cannot see the picture itself.

There are three audiences for alt text. First, people using screen readers, who hear the alt text announced in place of the image. Second, anyone on a slow or broken connection, who sees the alt text when the image fails to download. Third, search engine crawlers, which use alt text (along with the file name and surrounding content) to index images and understand page context.

Why Alt Text Matters for SEO and Accessibility

From an accessibility standpoint, alt text is a requirement, not a nicety. The Web Content Accessibility Guidelines (WCAG) success criterion 1.1.1 requires a text alternative for all non-text content. Images without alt text are a frequent cause of failed accessibility audits and can expose organizations to legal risk under laws such as the ADA in the United States and the EAA in Europe.

From an SEO standpoint, alt text is how images earn visibility in Google Images and contributes signals to the page's overall relevance. Descriptive alt text on a product photo, infographic, or diagram helps search engines connect that asset to the right queries. It also improves the experience for the growing share of users who browse with assistive technology — and Google has repeatedly said it rewards genuinely accessible, helpful pages.

Missing vs Empty vs Bad Alt Text

Not every alt issue is the same, and a good checker distinguishes between them:

• Missing alt — the attribute is entirely absent. Screen readers may fall back to reading the file name aloud (e.g. "I-M-G underscore 4821 dot jay-peg"), which is useless. This is the highest-priority fix.
• Empty alt (alt="") — an intentional empty value that tells assistive technology to skip the image. This is correct for purely decorative images like dividers, spacers, and background flourishes, but a problem when used on meaningful images.
• Filename alt — the alt is just the file name, such as "IMG_4821.jpg". Technically present, practically worthless.
• Redundant alt — text that starts with "image of" or "photo of". Screen readers already announce the element as an image, so the prefix is wasted.
• Too-long alt — descriptions that ramble past roughly 125 characters get truncated by some screen readers and bury the point.

How to Write Good Alt Text

Effective alt text is specific, concise, and contextual. Follow these rules:

A simple test: read the alt text aloud and ask whether someone who cannot see the image now understands why it is on the page. "Logo" fails that test; "SemlyPro logo, links to homepage" passes it.

How to Use an Alt Text Checker

1. Paste your HTML

Open the page in a browser, right-click and choose "View Page Source," and copy the markup into the checker. You can also paste a single section, a CMS template, or a block of generated HTML — the tool scans every <img> it finds.

2. Review the flagged images

The checker lists every image with its src and alt value, then tags each one as missing, empty, too long, filename, redundant, or good. Sort by issues to see exactly what needs work first.

3. Check your coverage score

The alt-coverage pass rate shows what percentage of your images have usable alt text. Aim for 95%+ (an "A" grade), counting correctly-marked decorative images as passing. Track this over time as you publish.

4. Fix and re-check

Apply the per-image suggestions, paste the updated HTML, and confirm the score climbs. Because the checker is instant and deterministic, you get the same result every time with no waiting.

Common Alt Text Mistakes

• Leaving the alt attribute off entirely instead of writing alt="" for decorative images.
• Using the raw file name as the description.
• Keyword-stuffing alt text in the hope of ranking — this hurts both accessibility and SEO.
• Writing alt text for icons that are already labeled by adjacent visible text (creating duplicate announcements).
• Forgetting alt on images inside links, which leaves screen-reader users with an unlabeled link.