Possible to check W3C validity easily from your local machine?

I am developing a site that I would like to check for validity with W3C with respect to HTML, CSS, and so on开发者_开发百科.

Because I am new to this, I am quite sure it will take an inordinate amount of trial and error to constantly attempt improvements, push them to the site, run the W3C test, then iterate. Is there a way to check validity from my local machine?

Second, how can I get W3C to check the validity of pages that require authentication?

You can use the "validate by direct input" service. Just view your page source on your local machine (whether authentication is required or not), copy and paste into the text box and check out your errors.

Edit:

I guess I could have included a link: http://validator.w3.org/#validate_by_input ;)

Depending on the browser and operating system that you are using, Firefox for Windows has a number of plugins that I have used in the past to validate markup. I haven't seen the same plugin for Mac/Linux. With a quick search, I also found something similar for Chrome.