Cross-browser CSS
Solution 1:
It's generally done by hand.
It typically starts by applying a CSS reset to even the playing field.
And you should aim at standards compliant CSS and then only as a last resort apply hacks for IE (yes I just called it out).
Solution 2:
For tools & services i tried Browsershot and Spoon:Browsers. Also for tips and tricks, i found this article and this helpful.
Solution 3:
WYSIWYG editors are very rare in the professional world.
Toolkits like YUI, jQuery, etc... are very popular since they offer a lot of cross-browser Javascript functionality.
However, for CSS, twiddling by hand with lots of swearing to get IE6 to work is common. Websites like quirksmode can help you out by pointing out browser-specific quirks.
Solution 4:
Most people just design to a common baseline: Firefox and/or Safari and Chrome. under most circumstances, the design will then Just Work with everything except the various versions of IE, for which one simple set of custom rules will fix.
Solution 5:
I think they pretty much have learned css compatibility across browsers. There are a multitude of charts on the internet that explains whats supported on each browser in detail and how it affects the rendering of the page.