HTML semantic outline checker

The scanner is a plain tag reader, not a browser. It does not run scripts, load stylesheets, or see shadow DOM, so hidden state is judged only from the hidden attribute, aria-hidden, and inline display or visibility styles, and heading levels are taken at face value exactly as assistive technology reads them. A paste without an html element is treated as a fragment, and the lang check reports that it could not run rather than guessing. This tool does not check alt text, contrast, labels, or color; the accessible name debugger and keyboard navigation auditor cover parts of that.