What does a validation engine use to determine the W3C standard it should validate against in a Web site?

The determination of the W3C standard that a validation engine uses to validate a website is based on the HTML page's !DOCTYPE declaration. The !DOCTYPE declaration is a crucial element that informs the browser and validation engine about the version of HTML or XHTML used in the document. By specifying the document type, it sets the rules for which standards and technologies are applicable during validation.

When a page includes a specific !DOCTYPE declaration, it signals to the validation engine which W3C specifications to apply when checking the page for compliance. For instance, a declaration for HTML5 will tell the validator to adhere to HTML5 standards, while an XHTML declaration informs it of different rules for XHTML documents. Thus, the !DOCTYPE declaration plays a vital role in ensuring the document is validated correctly according to the relevant standards, ensuring proper rendering and functionality across different web browsers.