HTML is the base language of webpages. The 4.01 version of the HTML language is a specification which has been set after years of discussion by the World Wide Web Consortium (W3C), an international consortium that involves the top 500 IT corporations in the world.
The HTML 4.01 specification does not provide formal error recovery rules: that means that a webpage with malformed or improperly coded HTML code can be rendered in various and unpredictable manners by web browsers and web-aware softwares. Incorrectly coded webpages may be rendered differently in web browsers which can impact the page layout. Therefore, the best starting step to ensure a webpage appears and works as intended in all modern web browsers and web rendering softwares is to write HTML code adhering to the HTML 4.01 specification. Generally speaking, a webpage using valid HTML code along with valid CSS code is smaller in size (so downloading is faster) and is also rendered considerably faster in modern browsers.
The W3C provides a public online HTML validator service that automatically checks a submitted webpage against most of the formal rules (semantic, syntax, document structure) of the HTML 4.01 specification and then reports any error found. The clickable W3C HTML 4.01 button-image at the bottom of webpages provides a way for any visitor to check whether such webpages' HTML code complies with most of such formal rules.
HTML validation does not mean that the webpage is an overall good web page or a well designed one. HTML validation does not guarantee that the webpage complies completely or perfectly with the whole HTML 4.01 specification. But, for softwares running on machines, parsing and processing the HTML code of webpages and then rendering webpages, this step is definitely the first best step toward creating well coded and well designed webpages.
More explanations on markup validation and HTML validation:
CSS is a language dedicated to formatting web pages and to specifying appearance, colors, position, dimensions, alignment, formating, etc. of elements in webpages. The CSS language has specifications which have been set and have been agreed after years of discussion by the World Wide Web Consortium (W3C), an international consortium that involves the top 500 IT corporations in the world. The latest official specification of the CSS language is CSS 2.1.
Unlike HTML 4.01, the CSS 2.1 specification has formal parsing rules and error recovery rules. So when there is a problem with the CSS code, it often impacts the layout and styling of the webpage in visual browsers and in the rendering of web-aware applications.
The W3C provides a public online CSS validator service that automatically checks the CSS code of a submitted webpage against all of the formal rules (list of properties, syntax, grammar) of the CSS 2.1 specification and then reports any error found and warnings. The clickable W3C CSS button-image at the bottom of webpages provides a convenient way for any visitor to verify that the CSS code in such webpages complies with such formal rules.
CSS validation does not mean that the webpage is an overall good webpage or a well designed one. But, for softwares running on machines, parsing and processing accurately-specified CSS code, this step is definitely one first necessary step toward creating well coded and well designed webpages.
More explanations on HTML validation and CSS validation:
Every page that has a clickable W3C WCAG 1 A button-image indicates a claim of conformance to the Level A of the W3C Web Content Accessibility Guidelines 1.0.
Web Content Accessibility Guidelines (WCAG) provide a list of checkpoints for various levels to improve the accessibility of webpages. For example, level A refers to the set of Priority 1 checkpoints. These accessibility guidelines and checkpoints were also developed within the W3C activities.
The purpose of creating accessible webpages is to increase the scope, the reach of webpages under various contexts and conditions, to improve the user experience, to increase the potential of the audience, for the benefits of a larger audience.
Sometimes, a more accessible webpage is just a page that achieves normal things or things one would reasonably expect from webpages.
Accessibility on the web is getting more attention these days as accessibility laws are passed in more states and countries (USA, United Kingdom, Australia, France, New-Zeland, Dutch, Canada, etc.). Courts of justice furthermore agree with defendant parties and accessibility organizations representing them.
More explanations on accessibility validation from :