QUOTE(t-a-g @ Sep 6 2006, 06:45 PM)
For HTML the important thing is to make it validate. Use any of these tools: http://htmlhelp.com/tools/validator/ http://validator.w3.org/
Also, check that the HTML elements are supported or at least degrade gracefully. See http://www.blooberry.com/indexdot/html/tagindex/all.htm
CSS is usually more bug-prone and less supported than HTML, so see also http://www.blooberry.com/indexdot/css/propindex/all.htm
Here's Microsoft's HTML, CSS and DHTML reference for IE/Win: http://msdn.microsoft.com/library/default....rence_entry.asp
What DTD should I start with?
DTD "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"
better or easier than
DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Browsers will normally not notice the difference between XHTML or HTML, I recommend HTML4 Strict, see http://www.htmlhelp.com/tools/validator/doctype.html
since HTML4 is much simpler and XHTML is mostly hype with no real advantages.
For especially CSS it's important wether the Doctype puts the browser in quirks- or standards mode. I recommend standards mode, see http://hsivonen.iki.fi/doctype/
This site is very useful: http://www.quirksmode.org/
AFAIK Mozilla/Firefox' rendering engine (Gecko) is the same on all platforms. IE/Win and IE/Mac are not the same. Not sure about Opera.
It's best to test continuously in all the major browsers from the start, rather than making a whole site in say Firefox and only test in IE afterwards.
If you don't have access to a particular browser a screen capture service might help: http://www.google.com/search?q=browser+screen+capture
however this can be tricky with DHTML that requires user interaction.