# HTML Nesting Validator

- Human page: https://happytails.ai/en/developer-tools/html-nesting-validator/
- Markdown: https://happytails.ai/en/developer-tools/html-nesting-validator.md
- Structured JSON: https://happytails.ai/agents/en/developer-tools/html-nesting-validator/index.json
- Visibility: public

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `html-nesting-validator` |
| Category | developer-tools |
| Implementation | Registered implementation. Registration alone is not a production-quality guarantee. |
| Browser execution | Use the visible action or interactive controls. |
| Browser processing | Browser; see tool notes for network behavior. |
| HTTP API | Not callable through HTTP. |
| Browser-agent interface | run_current_tool |

### Implementation notes

Check common HTML tag nesting errors locally.

### Inputs and settings

Text input: HTML source.

Example or placeholder shown in the interface (not a validated fixture):

```text
<p>Hello <strong>world</strong>.</p>
```

Paste HTML markup, not a website URL. Up to 200,000 characters. Review the settings before running.

No additional settings are declared for this tool.

### Browser-agent workflow

1. Open the human page in a browser that supports WebMCP.
2. Discover the interface exposed by that page; use its actual schema.
3. Select files on the page first when the tool requires files.
4. Call `run_current_tool` with valid input and settings.
5. Inspect the returned result or error and the displayed output. Retrieve files from the displayed download links.

Calls do not automatically copy or download results. Browser permissions still apply. Interface availability alone is not proof of successful execution.

### Planning specification

Input is limited to 200000 characters. Results are computed locally and exported as JSON.

This describes intended requirements. Use the implementation notes, interface schema and observed output to determine current support.

### Complete machine-readable capability record

```json
{
  "id": "html-nesting-validator",
  "category": "developer-tools",
  "built": true,
  "requirements": "Input is limited to 200000 characters. Results are computed locally and exported as JSON.",
  "implementation": {
    "fields": [],
    "inputMaxLength": 200000,
    "inputErrorCode": "STRUCTURED_TEXT_TOO_LONG",
    "registrationFamily": "html-nesting-validator",
    "note": "Check common HTML tag nesting errors locally.",
    "experience": {
      "automatic": false,
      "single": false,
      "inputMaxLength": 200000,
      "inputMode": "text",
      "label": "HTML source",
      "placeholder": "<p>Hello <strong>world</strong>.</p>",
      "help": "Paste HTML markup, not a website URL. Up to 200,000 characters. Review the settings before running.",
      "action": null
    },
    "mode": "explicit",
    "interactive": false
  },
  "execution": "Browser interface",
  "api": null,
  "browser_agent": {
    "supported": true,
    "name": "run_current_tool",
    "discovery": "WebMCP on the human page in a supporting browser",
    "verification": "See audit/API-AUDIT.md; availability is not verification"
  }
}
```

## Page guide

Find mismatched closing tags and obvious invalid HTML nesting locally. Runs locally and returns a reviewable report.

How can we improve this tool?

Report a problem or share an idea.

Please leave out private information. Your message and the tool name go to our team via PostHog.

### How to use HTML Nesting Validator

1. Paste the HTML.
2. Run the local check.
3. Review tag-stack issues and download the report.

## Tag structure

The validator checks a bounded tag stack. Browser parser repairs, semantics and accessibility require separate review.