The process of validating markup is used to ensure the overall correctness and professionalism of the site. That being said, incorrect or outdated markup can make future site updates and html code deprecation problematic.
Examples of bad markup:
- Non-closing tags
- Missing alt attributes on img tags
- Multiple h1 tags within a section
- Empty tags
- Images without height and width attributes
This process is really all about housekeeping, making sure that that everything has been ticked off and coded properly. Even the best web developers can miss things out, which is why it’s important to check that everything is as it should be. Most code editors will point out obvious errors, but it goes without saying that errors can still be missed.
Obviously one of the go to places to check HTML markup would be the W3C, so here is a link to their online validator, http://validator.w3.org/.