First, because one leads to another. A standards-compliant website will, most of the time, be an accessible website... As simple as that.
Second, because not everyone is lucky enough to have the latest Internet browser installed on the latest OS on the latest top of the line machine, with a 30" monitor and fiberoptic connection. Not to mention that there are many unfortunately disabled by nature, such as blind or paralyzed people, who cannot use the machine in the way that is thought to be 'usual'. But they all have the right and need to access the Internet and be able to interact with it easily and productively.
Putting it simply, it means that all the markup used to build every page of a web site is "grammatically" and "orthographically" correct when compared to the standards recommended and/or required by the World Wide Web Consortium.
Of course, "grammar" or "orthography" or "punctuation" are not the most appropriate terms to refer to the structure of the markup, but - by association with any other human language (and keeping in mind that this is a layman's terms guide, peeps) - there are similar aspects. For example, a "grammatically" correct markup will only contain tags that have a structural purpose (i.e. elements that serve as containers for the various contents of the document) and will not contain tags that have to do with the aesthetic aspect of the contents (i.e. the ancient <font> or <center> tags, which are deprecated). On the same note, an "orthographically" correct markup will only contain tags writen in lower case, elements containing both the start tag and the </end> tag, all correctly nested and so on.
As a conclusion - and an answer to the inevitable "but how do I add some eye candy to my pages?" question - a standard compliant markup should be able to be described by the following attributes:
[For more details, see the links below]
Simply, any user - including people with disabilities - using any kind of Internet browser on any kind of machine or device at any connection speed gets to view the web page in good condition, interact with it easily and without any obstacles.
A strong relationship is being developed between standards compliance and accessibilty, in that today's web design standards are, in their majority, rules and guidelines that - together with the proper response of the user agent - ensure the user-friendly, accessible character of the Internet contents.
<img src="../images/image_name.jpg" alt="short description of the image" width="x" height="y" />
<img src="../images/image_name.jpg" alt="short description of the image" title="additional info can be given through the title attribute (tool tip box, which appears when MouseOver the image)" longdesc="link (the URI address) to an external document providing a long description of the image" width="x" height="y" /><script type="text/javascript">
<!-- a Javascript string for whatever purpose you might need //-->
</script>
<noscript>Alternative contents (such as a link to a page that doesn't use Javascript or a warning message that appears when a browser that cannot process Javascript accesses the page) for users that cannot view Javascript controlled contents</noscript>
<iframe src="external_document.html" name="frame" scrolling="auto">short description of the frame purpose/function</iframe>
<noframes>alternative contents (such as links to pages without frames, target="_blank" type of links towards the documents that usually appear inside the frame etc...) for the users that cannot/or chose not to allow framesets to be used - in their browsers</noframes><table summary="short description of the table contents">
<caption>short description of the table function or contents</caption>
<tbody>
<tr>
<th>title of the column, if applicable to the contents</th>
·
·
·
</tr>
</tbody>
</table><h1>The Page Title<h1>
·
·
<h2>Subtitle<h2>
·
·
<h3>chapter title<h3>
·
etc...
Do not use the hn element as aesthetic tool (eg. big fat lettering). Let the style sheets give color and style to the contents.<p>For easier reading or separation between various ideas etc., use the paragraphs the way they use them in books and other printed documents...</p>
<p>There is no lower or higher limit for the number of words or lines in a paragraph, it is absolutely flexible to meet your needs. However, do not use paragraphs to contain images only or titles</p>...
<p>Page title</p> <-- incorrect use
<p> </p> <-- incorrect use
<p> </p> <-- incorrect use
<p> </p> <-- incorrect use
<p>Contents etc...</p>[For more details, see the links below]