Paste your HTML and the checker reads it tag by tag, then lists the mistakes that break pages: tags that are never closed, tags closed in the wrong order, duplicate ids, images without alt text and missing page basics such as the doctype, language, title and viewport. Each row gives the line number and a short fix. It checks only the code you paste and is not the official W3C validator.
Browsers never refuse to show a page because of bad HTML. Instead they follow a repair procedure that is written into the standard: close a paragraph when a block element starts, move stray table content out of the table, and close open tags when their parent ends. The page appears, but the repaired tree can differ from the one you meant.
That is why broken markup produces odd symptoms rather than clear errors: a menu that shifts down a line, a style that applies to the wrong block, a form that submits only half of its fields. Finding the tag that started the problem is faster with a checker than with the browser's inspector, which shows the repaired result and hides the original mistake.
Duplicate ids come first on the list because scripts and labels trust them. A label that points to an id used twice attaches to the first match, and a script that looks up a duplicate returns the wrong element. Unclosed and mis-nested tags come next because they change the page structure.
Missing basics come last but still matter. The doctype keeps browsers out of quirks mode, the charset tag stops accented letters turning into garbage, the viewport tag makes the page fit a phone, and the title is the first thing searchers read. Alt text on images helps search engines and everyone who cannot see the picture.
No. It is a lightweight checker that runs in your browser and looks for the most common mistakes. It does not know every rule in the HTML standard, so for a formal check run the official validator as well. It is faster for day-to-day debugging because there is nothing to upload.
It means the tag was opened and never closed before the file or its parent ended. For example, in <div><b>text</div> the b tag is still open when the div closes. Browsers guess how to repair this, and the guess may not be what you intended.
The HTML standard lets a few elements, including p, li, dt, dd, tr, td, th, option, head and body, end without a closing tag. The checker follows those rules, so a list item that is closed by the next list item is fine.
Tags that older HTML versions used for looks, such as font, center, marquee, big and strike. Browsers still show them, but the standard no longer allows them. Use CSS for colour, size and alignment instead.
No. The check runs entirely in your browser on the text you paste. Nothing is uploaded and no web address is fetched.
Related: HTML accessibility checker, Meta tag generator, Text to HTML converter.
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