# Attribute “xmlns:dt” not allowed here.
> Canonical HTML version: https://rocketvalidator.com/html-validation/attribute-xmlns-dt-not-allowed-here
> Attribution: Rocket Validator (https://rocketvalidator.com)
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
The `xmlns:dt` attribute — short for "XML Namespace: datatypes" — was historically used in Microsoft-specific XML vocabularies (notably `urn:schemas-microsoft-com:datatypes`) to declare data type information on elements. It was common in older ASP and IE-era markup. However, HTML5 is not an XML language, and it does not support arbitrary XML namespace declarations on elements.
In HTML5, the only `xmlns` attribute permitted is `xmlns="http://www.w3.org/1999/xhtml"` on the `` element itself, and even that exists solely for compatibility with XHTML serialization. Namespace-prefixed attributes like `xmlns:dt`, `xmlns:o`, or `xmlns:v` are invalid. The HTML parser simply does not recognize them, and the W3C validator will flag them with the error **"Attribute 'xmlns:dt' not allowed here."**
## Why This Is a Problem
- **Standards compliance:** Using non-standard attributes means your document does not conform to the HTML specification, which can lead to unpredictable behavior across browsers.
- **Legacy lock-in:** The `xmlns:dt` attribute is tied to Microsoft's proprietary data type schema. Modern browsers do not interpret or use this namespace, so it serves no functional purpose in an HTML5 document.
- **Validation noise:** Invalid attributes generate validator errors that can obscure real issues in your markup, making it harder to catch genuine bugs.
- **Accessibility and tooling:** Screen readers, search engine crawlers, and other automated tools expect valid HTML. Non-standard attributes can confuse parsers or be silently discarded.
## How to Fix It
1. **Remove the attribute.** If `xmlns:dt` was carried over from legacy code or a CMS template and nothing in your application depends on it, simply delete it.
2. **Replace with `data-*` attributes.** If you need to attach custom metadata to an element — for example, to indicate a data type for use by JavaScript — use an HTML5 `data-*` attribute instead.
3. **Use XHTML if XML namespaces are required.** If you genuinely need XML namespace support (rare in modern web development), serve your document as `application/xhtml+xml` with a proper XHTML doctype and XML declaration. Be aware this changes parsing rules significantly.
## Examples
### Incorrect: Using `xmlns:dt` in HTML5
This will trigger the validation error:
```html
Item one
42
```
Both `xmlns:dt` on the `
` and the `dt:dt` attributes on the `
` elements are invalid in HTML5.
### Correct: Attribute removed
If the namespace declaration is unnecessary (which it almost always is in modern HTML), remove it along with any prefixed attributes:
```html
Item one
42
```
### Correct: Using `data-*` attributes for custom metadata
If your JavaScript or application logic needs to know the data type of each item, use valid `data-*` attributes:
```html
Item one
42
```
You can then access these values in JavaScript with `element.dataset.type`.
### Correct: Migrating a legacy `div` wrapper
A common legacy pattern places `xmlns:dt` on a container `div`:
```html
2024-01-15
```
The fixed version removes the namespace and uses standard attributes:
```html