why I need w3c valid html and css?

[alok.jain]

1. What are the benefits of using W3C Valid HTML and CSS pages.

2. What are the qualities of a good web page?

[Joe Schmoe]

1) Is your website commercial? Do you want to use it to make money? If so, then using W3C Valid HTML ensures that you exclude absolutely no customers. Would you build a store and require you customers to only drive compacts to get in the parking lot? I use that example because some places do this. That's exactly what you do when you design web pages for specifc browsers excluding others.

2) Well... everyone would give different answers for this. But I'd say, easy of navigation, pleasant to the eye and consistency.

[leighjam]

First off the benefits of Valid HTML/CSS is that in theory it SHOULD be rendered the same on every screen. This makes your site look great on EVERY page. In addition spiders and other automated visitors to your site can glean info a LOT easier and faster.

Unfortunately due to pride, ignorance or something else, IE (version 8 may be an exception), Firefox (Versions up to 2) which make up the majority of browsers don't always follow the rules. So you may have to 'break the rules' to make something work consistently.

The BEST thing to to is code valid HTML/CSS THEN place the fixes in after. If at all possible through browser detection.

Some qualities of a GOOD webpage outside of looking pretty.
* Small Download Size
* Clean Text
* No more than 3 fonts and font sizes.
* Clear optimized images.
* Clear Navigation
* Alternative text/content for Images, Flash and other types of graphical objects.

[lababy24]

Quality of good web page is that it can work on almost any platform or browser, it's easy to navigate and the colors are easy on the eyes, also that it has a decent navigation so that the user know where is at only no more than three clicks to get where it needs to go.
W3C was implemented with the idea that the pages can be seen on other devices like ipods, cell phones, and also on regular size computers with little or not distortion, since little devices can't process all html tags as a computer would. it was also created with the goal to use more css to format pages. I have a degree on web design.

[vftw.com]

In addition to the other answers,

1. Following standards compliance also ensures longevity of your site. It makes it future proof to an extent. If we're talking about separating style from content using CSS, it also helps non graphical browsers, such as braille and audio browsers. If you are making sites for large companies, this can actually be a legla requirement.

2. A good web site should also follow Web Accessibility Guidelines - http://www.w3.org/WAI/ - In doing so you eliminate most technical reasons for people not being to access your web site, no matter what browsing technology they use.