How does the W3C Markup Validation Service determine to what HTML standard to validate your code?

• A. It reads the DOCTYPE declaration on an HTML page and validates according to the specified DTD.
• B. It uses the file extension to decide the standard.
• C. It uses the URL of the page to determine the standard.
• D. It always validates against HTML5.

Answer: (B)

The main idea being tested is how the validator chooses which HTML standard to apply. It does this by reading the DOCTYPE declaration at the top of your document. That DOCTYPE tells the validator which specification and version to validate against (for example HTML 4.01, XHTML 1.0, or HTML5). If you use the HTML5 DOCTYPE, the validator applies HTML5 rules. File extensions or the page’s URL don’t determine the standard, and the validator isn’t limited to HTML5 for every document—it adapts to the DOCTYPE you provide. If there’s no DOCTYPE, some validators may default to HTML5 mode, but including the correct DOCTYPE is best practice for accurate validation.