How to fix HTML errors?

I would perhaps try the transitional XHTML Doctype to start with instead of your current one and see if that reduces the errors somewhat.

I've never seen CSS declared with markup tags like this before. Perhaps someone could advise you on its validity.

Eg.

As I said in the other thread, change your document type to html 4 transitional and you have only 150 errors to deal with. That's nearly four fifths of your errors gone.
It really doesn't matter which you use unless for some reason you need a specific one. I only use XHTML mostly as I code that way anyway.

Some people argue that validating means nothing, but it is untrue. While your site being valid doesn't necessarily make it better, work 100% or even look the same in all browsers, it does at least ensure you have correct HTML and CSS code.

It isn't the end of the world if you have errors, but fixing them teaches you to write good HTML in my opinion. It shows common mistakes and things you shouldn't do (to a point of course).

The doctype is the declaration you make at the top of a file that declares that file is to be used for a HTML document.
Info about Doctypes and how to change yours

Once you change it, go back to W3C and try to fix as many errors as you can. You will be able to fix most of them fairly easily, and some not so easy.
Some will be tricky to fix as you use third party code and plugins, like Twitter, and often their code doesn't validate.

But the more you fix errors, the more you learn about how to write better HTML, and the more chance you site has of pleasing search engines.
It's important to note that your site validating with W3C doesn't necessarily make SE's like the site any better, they will do that if your code is written correctly, and validation just helps you do that.