This article discusses what validation means, points you to some of the free tools that you can use, and deals with its limitations and the problems that a new webmaster may face. Otherwise you'll be completely lost here since I assume you at least know what these terms mean.Note: if you are not sure what HTML and CSS mean, please read What are HTML, CSS, Java Script, PHP and Perl? For those unfamiliar with the term, "validating" a page is just a jargon-filled way of referring to the use of a computer program to check that a web page is free of errors.That is, it will check that it complies with the CSS standards set by the W3 Consortium.There are a few which will also tell you which CSS features are supported by which browsers (since not all browsers are equal in their CSS implementation).I try to validate my pages each time I make modifications, although I must admit that I sometimes forget to do so (with the occasional disastrous consequence; Murphy's Law doesn't spare webmasters).I find that having an offline validator helps to make sure that I remember to validate: having to go online just to validate my pages tends to make me put off validation till later, with the result that it'll occasionally get overlooked.More troubling, the error messages it gives you are sometimes very mysterious and not very helpful.
So if you write HTML code that has (say) the wrong order, the HTML validator will spot it and tell you.
For those not familiar with the terminology I use, when I say "offline validator" I simply mean a validator that I can download and install in my own computer so that I can run it on my pages without having to go to the W3 Consortium's website.
You can find offline validators on the free validators page I mentioned earlier, that is, https:// The HTML Tidy validator (listed on that page) is available for numerous platforms (including Linux, Mac, Windows, etc) and has proven helpful to many webmasters the world over.
If you are wondering what the difference is, an analogy from normal human language will hopefully make it clear.
Let's take this sentence "Chris a sandwich ate" which is grammatically incorrect when used in a non-poetic context.
Depending on the complexity of your code, you may even want to test it with different browsers to make sure that your site looks the same in all of them, since it's possible that you are using features of HTML and CSS that are only implemented in some browsers but not others.