What does validating a web page mean
That might have been the case a decade ago, when validation was the tool of choice of people more interested in harnessing the power of the markup languages than creating beautiful designs for their content; when many designers were not taught basics of Web technology and would create beautiful but fragile and unreliable web sites. In the past decade, most of the stunning, content and design-rich Web sites were built with standard (X)HTML, CSS and scripting.
One of the discussions that arises from time to time in web development circles is that of validating your code. It’s no different than comparing a sentence you write to the rules of grammar for the language you wrote it in.
When surveyed, a large majority of Web professionals will state that validation errors is the first thing they will check whenever they run into a Web styling or scripting bug.
Checking that a page “displays fine” in several contemporary browsers may be a reasonable insurance that the page will “work” today, but it does not guarantee that it will work tomorrow.
The original version was written by Nick Kew of WebÞing Ltd.
Note that I also left off the period after “This is a sentence” above, which violates the rules of grammar, but likely doesn’t cause any problems in communication.
This is deliberate, and doesn't imply any kind of browser bug.
A term sometimes used for this is WYSINWOG - What You See Is Not What Others Get (unless by coincidence).
Creating Web pages or applications according to a widely accepted coding style makes them easier to maintain, even if the maintenance and evolution is performed by someone else.
Many professionals have been authoring the Web with HTML and CSS for years and know these technologies by heart.
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For example This is a sentence validates under html 4.01 transitional or html5 where the closing tag is not required, but not under xhtml 1.0 transitional where the closing tag is required.