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.
CSS or cascading style sheet is a web based application language which describes the presentation and style from the document that is written in a markup language.
Most of the internet pages are written in HTML and Extensible HTML language.
Mark-up validators cannot see the "big picture" on a web page, but they excel at picking up missed closing tags and other technicalities.