What does the HTML Preview and Structural Validator do?
Use this tool when you need a browser utility that renders pasted HTML in a script-blocking sandbox and checks common document-structure and accessibility mistakes. It accepts an HTML document or fragment and returns a sandboxed preview and local diagnostics for document, image, ID, and form-label structure. Results are deterministic for the same input except where cryptographic randomness, current time, or a live provider response is part of the task.
Use local preview for fast feedback, Nu for conformance
This page is suited to safe visual inspection and common structural mistakes. It does not claim complete HTML or accessibility validation.
How do you use it?
- Paste an HTML document or fragment.
- Choose Preview for sandboxed rendering or Validate structure for deterministic checks.
- Fix reported document, duplicate-ID, image-alt, or form-label issues and rerun.
Worked example
Find a missing image alternative
Input
<main><img src="photo.jpg"><p>Hello</p></main>Output
Warning: 1 image is missing an alt attribute.The local DOM inspection catches a common accessibility defect without loading a remote validator.
Common errors and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Scripts do not run in the preview. | The iframe intentionally has a restrictive sandbox. | Use a controlled development environment when script execution is required. |
| No warning appears for a standards issue. | The tool performs focused structural checks, not full conformance validation. | Use the Nu HTML Checker for release-grade standards validation. |
Important behavior of HTML Preview
These deterministic checks are not a substitute for the complete WHATWG conformance rules or the W3C Nu validator.
Standards and implementation references
- WHATWG HTML Living Standard (opens in a new tab)Defines HTML parsing and document conformance.
- Nu HTML Checker (opens in a new tab)Provides comprehensive standards-oriented HTML checking.
Frequently asked questions about HTML Preview
Does the HTML Preview upload my input?
No. Processing runs in the current browser tab, and the page does not send tool input to a KitLumi processing endpoint.
What input does the HTML Preview accept?
It accepts an HTML document or fragment. The workspace checks the input and reports malformed or unsupported values before it produces a result.
What does the HTML Preview produce?
It produces a sandboxed preview and local diagnostics for document, image, ID, and form-label structure. Copy and download controls appear when the output format supports them.
What limitation should I know?
These deterministic checks are not a substitute for the complete WHATWG conformance rules or the W3C Nu validator.
What changed?
Added Sandboxed HTML preview, Duplicate ID detection, Image alt checks, Form-label checks, responsive controls, explicit runtime disclosure, and task-specific examples and limitations.