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Forum Moderators: not2easy
IMO the basic foundation, which I'd wished someone had shared with me, oh, 13 years ago - understand the following topics (with the exception of the validator, all links right in this site . . . .)
Next, validate your pages [validator.w3.org]. In your other thread you ask about broken FF/OK IE . . . if you use a full valid doctype and validate your pages, 95% of your cross-browser problems will go away.
Understand document semantics and use them. The tags you use - p h#, div, table - all have a semantic meaning that describes their content. There are a wide variety of threads here relating to this, the ones of largest debate are those relating to using tables for layout [webmasterworld.com]. A table is for tabular data, rows and columns of intersecting data, and using tables for layout confuses screen readers and "misrepresents" the content. This is the most glaring example of misuse of the semantic meaning of a page element.
This leads, almost naturally, to the separation of markup and content - that is, extracting things like this
<p style="color:red"><font face="arial">UGH</font></p>
into an external style sheet to control layout and style.
<p>Ahhh . . </p>
This is discussed deeply in the CSS forum [webmasterworld.com].
Separate content from markup and your document size gets smaller, loads faster, you expose your content to the search engines better, and can easily change the entire site without re-coding every single page.
Last and certainly not least - but often, most overlooked or ignored - is understanding accessibility and usability [webmasterworld.com]. The basic concept is every user - every one, not just those using certain browsers - should be able to access your site's information in an intelligible way.
People usually get their priorities out of order, addressing search engine positioning and site traffic before having these in place which, in my sector, is where I come in as "head janitor" cleaning up the mess . . . Start with these five things . . . and the others will follow . . . and can be found in the library [webmasterworld.com] on this site.