What does validating a website mean
Adding 50kb of download for each visitor because you didn’t stand up to your client? The reason was that they were using an unclosed – and rather obscure – HTML tag, the tag, but instead of rendering tags inside it, it outputs them.
Hard to guess why exactly the OP wanted the validator to run offline.
I know W3C has a site you can go to that will validate an HTML page (and count how many errors are found for a given doctype), but that has to be a publicly available URL.
How do you validate HTML on a locally running setup like I've described above?
Since I’ve taken it upon me to create articles for each question that I’ve answered more than once, let’s have a look at the pros and cons of W3C validation.
We’ll have a look at the pros and cons of W3C and web design first, before we dive into the SEO aspects.