W3C HTML Validator

The W3C Markup Validation Service

The following situations while validating:

• It's an error when using <img> tag without "alt" property, alt="" is necessary.
• Another common error occurs when including a URL which contains an ampersand (&), replacing & with "&amp;" is the only correct writing, so are the other special characters.

References WDG – Common HTML Validation Problems for more details.