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The W3C Markup Validation Service lets editors check web pages for conformance to HTML and XHTML standards. It is helpful for catching minor problems such as duplicate section names or citation IDs. Although most major browsers will tolerate many of the errors, and will display a document successfully even if it contains errors, they may misbehave on documents that contain the errors: for example, they may go to the wrong section or citation if you click the mouse on a wikilink. Checking that a page contains valid HTML can thus fix these minor glitches while also increasing portability to unusual browsers.
To check the HTML for a Wikipedia article XYZ, visit the service's home page and enter the text "https://en.wikipedia.orgview_html.php?sq=Facebook&lang=en&q=XYZ".