What is HTML markup validation?

Answer 1

HTML markup validation is a process by which the HTML standard DTD (Document Type Definition) is applied to an HTML document. If the document follows the rules laid out by the standard, the document is said to be valid. Should a document fail the test, it's deemed invalid.

This process is normally conducted by a computer program. Because the standards are produced by the W3C, their validator is the de facto standard, although, due to the open nature of the standard, other validators exist.

Invalid code causes the browser to go into "quirks mode" and makes how the browser renders the code a "best guess" situation. Producing valid code ensures that it will be treated in a standard way by all browsers. (In other words, at least you'll know what bugs you might run into with valid code.)