Specification mandates value for attribute

An attribute without a value, or a space inside a tag name.

The message, as the validator reports it

Specification mandates value for attribute disabled

Why it happens

HTML-style boolean attributes: <input disabled>. In XML every attribute needs ="…".
A space in an element name: <order date> reads as an element order with an attribute date that has no value.

How to fix it

Give the attribute a value (disabled="disabled"), or remove the space from the element name. Names may contain letters, digits, -, _ and ., but not spaces.

Example

Reports the error

<input disabled/>

Fixed

<input disabled="disabled"/>