Page Code Standards / Validation / XHTML Compliance

Web authors structure their pages using text, images, and multimedia content, then specifying what appearance, or style, the results should take on.

Every language, including those used for web pages, has its own rules for specific grammar, vocabulary and syntax. Current protocol specifies that all web documents should closely follow the pertaining appropriate rules.

However, web documents using these "Markup Languages" can easily contain a multitude of human-introduced typographical and carelessly created errors. The process of verifying whether a document actually follows the rules is called validation. A document that passes the validation process with success is referred to as valid or Compliant.

So Why Validate?

Browsers currently accept web pages and attempt to display them even if they're not comprised of validly compliant code. When encountering a page containing invalid code they will make "educated guesses" about what was intended. Various browsers (or even different versions of the same browser) will not all guess alike as to how to best correct for the error/s. Sometimes, little mistakes are negligible and the browsers pull off the corrections successfully. Other times, and with greater invalidity of code, the chances increase that the browser could become hopelessly confused producing a mangled mess, potentially even crashing.

Valid code increases the cross-browser compatibility which increases the likelihood that its' rendering will be as uniform as possible. Valid code also goes a long way toward debugging later problems as many are often caught early while it is being written. Valid and Compliant code is far more likely to be "future-proof" and not need revising to accommodate future browser version editions.

The best way to ensure validly coded sites is to run the sites' pages through a Validator, and/or to employ valid page templates.

Above paraphrased from: w3.org