W3C validation

What we check

We send the HTML of the home page to the official Nu HTML Checker of the W3C, the body that sets the web standards. It checks whether the code complies with the rules of HTML.

• More than 10 errors: serious defect.
• 1 to 10 errors: warning.
• Warnings of the validator: note.

In the report you see the first ten errors and warnings with line number and description.

Why it matters

HTML is the language in which websites are written. Like every language it has rules: elements have to be closed, certain elements may only appear in certain places, attributes have fixed spellings.

Browsers forgive a lot. They repair faulty code in the background and still display something. This has consequences:

• Different appearance: every browser repairs slightly differently. A forgotten closing element can stay invisible in one browser and wreck the layout in another.
• Unreliable reading: Google, AI crawlers and screen readers do not always process faulty code as expected. An incorrectly nested section may then be assigned to the wrong heading.
• Signal of the state of maintenance: many errors arise from outdated templates, plugins or copied code snippets. They show that the technical basis has not been revised for some time.

How to fix it

Find errors

Enter the website's address on validator.w3.org/nu. The full list shows every error with line number and the code affected.

Prioritise errors

Start with the errors that have the greatest effect:

1. Elements that are not closed or are nested incorrectly, e.g. a <div> inside a <p>.
2. Duplicate IDs (see separate article).
3. Missing required attributes, e.g. alt on images.
4. Outdated elements and attributes (see separate article).

Fix at the source

Most errors come from a few places: the theme, a plugin, an embedded widget. If the same error occurs 20 times, it can usually be corrected in a single template.

WordPress and builders

• Update theme and plugins: current versions often contain corrections.
• Check custom HTML blocks: code snippets inserted by hand, for example for widgets or tracking, are a frequent source of errors.
• With Wix, Jimdo and Squarespace the code comes from the provider. Little can be changed here, except for self-embedded elements.

How to check it yourself

On validator.w3.org/nu choose "Check by address", enter the website's address and click "Check". A green message means no errors.

Frequently asked questions

The page looks good. Why do errors in the code matter?

Browsers are very forgiving and silently repair faulty code, but each in its own way. What is right in Chrome can shift in Safari. And search engines, AI services and screen readers do not always repair in the same way.

Does the page have to be error-free?

Error-free is ideal, but not a must. What matters is to fix real errors such as elements that are not closed or duplicate IDs. Warnings are mostly pointers to outdated notations.