It is essential that the basic markup validates, i.e. no tags are left open, etc.
Other issues such as missing alt tags on images are less important (however, for usability, they should be present when meaningful).

It is arguably less important for mega-sites to validate since browsers will be tested during development on such sites, and if necessary, error handling will be tweaked to try to ensure they are displayed correctly. However, most errors are likely to be "best practice" rather than "critical".

Kaled.

Does anyone care about valid HTML?

Your visitors will never care whether your HTML is valid, or whether you use tables for layout . . .

unless . . . .

. . . . They access your site using a browser, resolution, or other variable environment that you've never experienced and your invalid HTML throws the document into Quirks mode, breaking the layout and creating a mess of it.

. . . . the invalid HTML is one of the errors that causes whole sections of a page to disappear (missing quote in an attribute, for example.) Some browsers "fix" this during rendering and it may go unnoticed.

Agreed.

And if/when you encounter a problem with your site which you need to troubleshoot, you'll be glad to have nice clean code to start with, rather than weeks of untangling various HTML errors.

Three green check marks all the way. Tis' the only way to fly.

You'll be glad to have nice clean code to start with, rather than weeks of untangling various HTML errors.

Indeed you will! That's a rather large part of the equation to knock out right off the bat.

When you see sites that have hundreds upon hundreds of HTML errors, many of those are cascading from a group of basic errors. Once they are addressed, the list of errors drops considerably.

Why don't Webmasters care about this? Because most others don't. But, there are a select group of professionals who can see the forest through the trees and are working toward validating their production code.

To me it's important that the documents I write is written in valid HTML 4.01 Strict and valid CSS. I care simply because I want usability and a close to perfect layout in most browsers.

In the past months, I have actualy rejected several link requests (I own a niche directory -- links to quality websites only), mainly because of invalid MarkUp: websites optimized for IE only, which looked horrible when viewed in FF or Opera.

Yes, I care about valid HTML, like I care about the content of my websites.

OK i have to indulge in a pet peeve of mine... There's a big difference between usability and accessibility.

Usability defines how easy it is for the visitor to use the site, e.g. can they find information easily, is the navigation straightforward and obvious, etc. Usability is mostly related to the site's presentation and programming and a valid HTML does nothing to improve usability.

Accessibility defines whether your site can be viewed and used at all by visitors with different abilities and disabilities, e.g. non-major browsers, the visually-impaired, the mechanically-impaired, etc. A valid HTML is a good step towards accessibility but it doesn't guarantee it.

There's also the concept of usefulness, a site can be usable and accessible without being useful if the content is not good, e.g. a search engine that returns links to irrelevant pages.

Three green check marks all the way. Tis' the only way to fly.

Sloppy work now will encourage even lower standards later. You'll have less issues arise, less complaints, and they will be easier to fix. Plus, clean code is so much easier to work with - and you can build a 'library' of solid layouts. A few tweaks to the CSS and the rendered (X)HTML is a 'brand new' design.