How to Check Your HTML With the W3C Nu HTML Checker

Your page loads, but that doesn’t mean the HTML underneath is doing everything it should. The W3C’s Nu HTML Checker can flag markup problems and point you toward the line that needs a closer look.

Quick answer: Open the checker, choose address or text input, enter a public URL or a safe HTML sample, and select Check. Read the messages, fix the source, then check again. A clean result is useful; it isn’t a complete accessibility, security, or quality audit.

We checked a public example page and a small HTML sample in a desktop browser. Then we corrected the sample’s missing image attribute and checked it again. We did not audit an entire website.

Choose what to check

1. Open the Nu HTML Checker.
2. For an already published page, leave Check by set to address, enter its full public URL, and select Check. The checker needs to be able to reach the page.
3. To practice with code, switch Check by to text input, paste a complete HTML example, and select Check. The page also offers file upload.

When you submit a URL or markup to an online checker, you’re sending it to that service. Don’t submit private pages, credentials, or confidential code without permission. A login-only or local page may not be reachable by URL; use an approved local workflow for sensitive work.

Read a message, fix the cause, recheck

In our sample, the checker flagged an img element without an alt attribute and showed the relevant line and columns. The screenshot captures that real result using a deliberately incomplete example.

We added an alt attribute to the sample and checked again; the checker reported no errors or warnings. That only tells us what happened with this small example. For a real image, choose an appropriate text alternative based on what the image communicates and its context. Some decorative images need an empty alternative; see the W3C’s image guidance rather than treating any arbitrary text as a fix.

For other messages, start with the first reported problem and inspect the surrounding markup. One mistake can cause later messages; fix the underlying source in your editor, republish when appropriate, and check the result again. The line number points into the document the checker received, which may differ from a theme template or source component.

Know what a clean result means

The checker tests markup rules, not whether visitors can use every feature. W3C explains that validation is not a full quality check. Test keyboard interaction, screen-reader behavior, functionality and security separately. A page can also change after a check, so rerun it after relevant updates.

For a separate visual check, try our guide to checking text color contrast; passing that check alone isn’t a complete accessibility audit either.

How we checked: We submitted example.org by address, then submitted a purpose-made sample with a missing image alternative. We added the attribute and reran the checker. The screenshot shows the first sample result; the interface is provided by W3C.