Editing W3C Markup Validation Service (section)

==Limitations==
Mark-up validators cannot see the "big picture" on a web page,{{clarify|date=September 2021}} but they excel at picking up missed closing tags and other technicalities.<ref name="W3C_CSS">{{cite web | title = About the CSS Validator | access-date=2012-06-24 | publisher=World Wide Web Consortium | url = http://jigsaw.w3.org/css-validator/about.html }}</ref><ref name="Castro">Castro, Elisabeth: ''HTML, XHTML & CSS, Sixth Edition'', page 345–346. Visual Quickstart Guides, Peachpit Press, 2007. {{ISBN|0-321-43084-0}}</ref>

DTD-based validators are also limited in their ability to check attribute values according to many specification documents. For example, using an HTML 4.01 DOCTYPE, <code>bgcolor="fffff"</code> is accepted as valid for the "body" element even though the value "fffff" is missing a preceding '#' character and contains only five (instead of six) hexadecimal digits. Also, for the "img" element, width="really wide" is also accepted as valid. DTD-based validators are technically not able to test for these types of attribute value problems.

Pages may not display as intended in all browsers, even in the absence of validation errors and successful display in other browsers. The only way to ensure that pages always display as intended is to test them in all browsers expected to render them correctly.<ref name="Castro"/>