HTML Accessibility Checker

Front-end developers preparing a release checklist, agencies running pre-launch audits, and content teams fixing templates all benefit from catching mechanical accessibility defects before a page ships. Paste a fragment or a full document, and this checker reports missing alt text, unlabeled inputs, unnamed buttons, heading skips, and related issues with the WCAG criterion and line number attached to each finding.

HTML Input

Findings

How the Static Audit Runs

The checker tokenizes the input into elements, attributes, and text nodes while tracking line numbers. Closing tags are recorded separately so nesting depth is accurate when the rule set looks for accessible names inside buttons and links, or for caption tracks inside video elements. Comments and doctype declarations are skipped without disturbing the line count.

Each rule is a pure function over the token stream. Image alt checks fire when the alt attribute is absent, and an explicit empty alt is treated as decorative and passes. Form label checks accept aria-label, aria-labelledby, a matching for attribute in any order, or a wrapping label. Heading order is compared as a sequence, and the first jump from one level to a level more than one higher becomes a warning.

Errors versus warnings

Errors are defects that block assistive technology with high confidence, such as a missing alt or an unlabeled input. Warnings mark patterns that often fail an audit but can be valid in context, like a click handler on a div that might be paired with keyboard code elsewhere in the bundle. Fix errors first, then review warnings with the surrounding component code.

Why line numbers matter

Findings sort by the line where the offending element opens, which matches the view-source editor most developers use. When a paste starts mid-document, line one is wherever you started, so add the offset mentally or paste from the top of the file. The copy button exports the same list as plain text for tickets and pull request comments.

Before and after: fixing a typical form block

Before (flagged)

<img src="chart.png">
<input type="password" placeholder="Password">
<button></button>

After (clean)

<img src="chart.png" alt="Q3 revenue chart">
<label for="pw">Password</label>
<input type="password" id="pw">
<button type="submit">Sign in</button>

Use Cases

Developer	Production	Workflow
Lint a component template before opening a pull request	Clear mechanical defects ahead of a third-party audit	Paste a CMS export and batch-fix editor blocks
Catch heading skips introduced by a page builder	Verify alt text coverage on marketing landing pages	Attach the findings copy to a Jira accessibility ticket
Review email HTML for labeled inputs and named buttons	Spot positive tabindex values before a UX review	Train content authors with the loaded sample document

Common Mistakes and Troubleshooting

Pasting only the inner markup of a page hides the html lang rule, because that check looks for the html element itself; include the document shell when you care about language declarations. Frameworks often assign accessible names at runtime through libraries, so a static paste can show a control-name error that disappears in the browser. Confirm against the rendered accessibility tree before filing a bug.

Angular and Vue templates use structural syntax such as *ngIf or :key that is not valid standalone HTML; compile or view-source the rendered output first. Large single-page dumps may contain several headings sections, and heading-skip warnings in one region are independent of another, so fix them region by region. Remember that duplicate-id warnings count exact attribute matches, including ones generated at runtime and pasted from the browser.

Best Practices for Accessibility Passes

Run the checker inside the component definition pipeline, not only on production HTML, so fixes land before release. Write alt text that conveys the function of the image rather than repeating the filename, and prefer real buttons over clickable divs so keyboard support comes for free. Keep one h1 per page and avoid skipping levels, because the heading outline is how many users navigate.

Pair this static pass with a color contrast check on the same components and a manual keyboard walk of the primary flows. Track remaining warnings in the same board as visual bugs so they get owners. When templates change in code review, paste the diff region back through the tool; a two-minute re-audit costs less than a failed compliance review.

HTML Accessibility Checker FAQ

What does this accessibility checker actually inspect?

The checker parses the pasted HTML fragment with a local tokenizer and runs a rule set covering missing image alt attributes, unlabeled form controls, controls without accessible names, heading level skips, missing html lang, duplicate IDs, positive tabindex values, click handlers on non-interactive elements, and videos without caption tracks. Each finding cites the WCAG criterion and the source line.

Why paste HTML instead of entering a live URL?

Browsers block cross-origin reads of other sites, so a static page cannot fetch and parse arbitrary URLs. Pasting the rendered markup, view-source output, or a template fragment keeps the tool honest about what it can check and keeps private staging content on your machine. Pair it with a crawler-based auditor for full-site sweeps.

Which WCAG level do the rules map to?

Errors map to Level A criteria such as 1.1.1 for text alternatives and 4.1.2 for name, role, and value. Warnings cover criteria that need context, such as 1.3.1 structure and 2.4.3 focus order, where the markup alone cannot prove a failure. The report lists the criterion identifier next to every finding so you can look up the full text.

Does a clean report mean the page is WCAG compliant?

No automated static check proves compliance. Keyboard paths, color contrast, zoom behavior, live regions, and real screen-reader output need manual testing. Use this report to clear the mechanical defects quickly, then run a keyboard pass and a screen-reader pass on the pages that matter most.

How are form controls matched to labels?

A control passes when it has a non-empty aria-label, an aria-labelledby reference, a label element whose for attribute equals the control id, or a wrapping label ancestor. Placeholder text alone does not count, because placeholders disappear on input and are not announced consistently by assistive technology.

What is the deal with duplicate IDs and positive tabindex?

Duplicate ids break label wiring and ARIA references because the browser resolves the first match only, so the checker warns with the count and first line. Positive tabindex values reorder the tab sequence away from the document order, which usually creates more confusion than it solves; the checker flags any value above zero.

Is pasted markup uploaded or stored?

No. Tokenizing and rule evaluation happen in the tab with local TypeScript. Staging HTML, client snippets, and unpublished templates never leave the browser, and there is no account, history, or network call in the audit path.