What are the reasons for validating a HTML?

Answer 1

By validating HTML against the W3C's published standards, you can assure that a browser that complies with those standards will be able to properly understand the code you have written.

Non-standard code causes modern browsers to go into "Quirks Mode." In quirks mode, the browser takes a best guess at what the author of the HTML document intended. Because this is just a guess, different browsers can arrive at drastically different interpretation of the non-standard code. Worse yet, a new version of the same browser might arrive at a different way of seeing the broken code.

Validation let's you rely (in theory) on the fact that standards compliant browsers will display standardized code the same way. It also ensures that future changes to the browsers themselves will not have a negative impact on your existing code.