Compatibility programs?

Compatibility comes from good valid code that adheres to standards. The only program that might come close would be Macromedia's Dreamweaver that isn't free or cheap.

The best thing you can do is to learn how to write XHTML and CSS by hand. As someone who has recently switched from using WYSIWYG to hand coding to standards, I can assure you that you won't be sorry!

*But* if you must........I think Mozilla's Composer would be your best bet for a free tool that does output decent code. It's part of the whole Mozlilla suite and you can get it at:
http://www.mozilla.org

For a plain html editor that offers a preview mode check out HTML Kit..they have both free and purchase versions:
http://www.chamisplace.com/

Another good tool is the CSE HTML Validator which has a free 'lite' version. This is both an html (text) editior and a validation tool.
http://www.htmlvalidator.com/lite/

But this isn't a good as the W3C's validation services:
Markup - http://validator.w3.org/
CSS - http://jigsaw.w3.org/css-validator/

You can demo a good CSS Editor called Topstyle:
http://www.bradsoft.com/topstyle/

You can also demo Dreamweaver MX.

Just about everything else I've tried or viwed the source of produced a lot of garbage code and garbage code will affect compatibility.

*Edit*
If you are asking if there is a tool that will magically fix existing code errors that affect compatibility......the answer is no. You'd need to run the code through a validator and by hand, fix the errors it tells you about.

Of course... even valid code can break on different browsers due to rendering differences in the browser engines.

Is there a magic wand that will automatically fix all of those problems? nope... you have to learn the differences in the browsers through trial and error. Good luck!

As andy206uk said, just because it's valid doesn't mean it will work

this for example
<script src="filename" type="text/javascript" />
is valid XHTML but will choke IE6, the fix is
<script src="filename" type="text/javascript"></script>

And I also believe Opera and Mozilla have margins and padding opposite to eachother

Until the day arrives when all browsers except the web strandards as one body, web pages will always be made up of bugs, issues, hacks and fixes.

Years ago, before I started writing in XHTML my sites always worked in IE and more often than not in netscape (IE5 and NS4.7 were my only browsers then), now I write only in XHTML IE is always a problem (mozilla now browser of choice). Almost as if MS are from another planet.

Also, while editors can be handy, they are a different story when editing server side pages, some (most) editors screw them up, from line breaks to adding weird stuff.

Ok ... What would be the best HTML coding standard to use? HTML 4.0?

XHTML 1.0 Transitional