Paste your HTML and instantly check for unclosed tags, missing attributes, deprecated elements, nesting errors, and other common markup issues — all client-side, nothing uploaded.
An HTML validator online parses your markup and checks it against the HTML5 specification rules. This validator checks for the most common and impactful issues: missing required elements, unclosed tags, deprecated markup, accessibility violations (missing alt), and invalid nesting. After validating and fixing your errors, you can paste the corrected code into our free html editor online to preview it in a live WYSIWYG view.
<div> or <p> can cause entire sections of a page to render incorrectly because the browser must guess the intended structure.<!DOCTYPE html>, browsers fall back to "quirks mode" — a legacy rendering mode with different behaviour for box model, tables, and positioning. Always include the DOCTYPE as the very first line.alt attribute is required by the HTML specification. Screen readers read it aloud for visually impaired users, and search engines use it to understand image content.<div> and <p> cannot be children of inline elements like <span> or <a>. Browsers auto-correct these violations in unpredictable ways.<font>, <center>, and <marquee> were removed in HTML5. Browsers still render them, but they should be replaced with CSS equivalents. Use our html cleaner online to strip deprecated tags and tidy up messy markup automatically.Googlebot and other crawlers parse HTML the same way browsers do. Malformed HTML forces the crawler to apply error-correction heuristics, which can misinterpret content structure, miss heading hierarchy, or fail to associate alt text with images. While Google is generally tolerant of minor markup errors, structural errors — especially unclosed tags that affect document flow — can cause content below the error to be ignored or classified under the wrong parent element. Valid HTML is one of the lowest-cost technical SEO improvements with consistent benefit.
No. The W3C Nu HTML Checker is a full-featured, server-side validator that sends your HTML to a remote server for processing. This tool runs entirely in your browser using JavaScript rules — nothing is sent to any server. It covers the most common and impactful errors quickly, but for a full conformance check against the complete HTML5 specification, use the official W3C validator at validator.w3.org.
Yes — all modern browsers have an HTML parser with built-in error-correction. This means a page with unclosed tags or invalid nesting will often still render visibly, but the resulting DOM may differ significantly from your intended structure. Different browsers may correct the same error differently, leading to layout inconsistencies. Writing valid HTML ensures the parsed DOM matches your intent across all browsers and tools.
Void elements are HTML elements that cannot have content and therefore don't have a closing tag. The complete list: <area>, <base>, <br>, <col>, <embed>, <hr>, <img>, <input>, <link>, <meta>, <param>, <source>, <track>, <wbr>. In HTML5, self-closing slash syntax (<br />) is optional; the validator does not require it.
Browser rendering engines are extremely forgiving — they apply error-correction rules defined by the HTML5 parsing algorithm, so visually the page may look fine. Validation errors still matter because: (1) error-correction behaviour differs between browsers and may cause inconsistencies you haven't noticed; (2) screen readers and assistive technologies rely on a correct DOM; (3) crawlers and scrapers may interpret the corrected DOM differently from what you intended; (4) future browser updates may change error-correction behaviour.