Looking for suggestions about what's the best way to make sure the addon looks the way you think it should look in multiple browsers. Looking for compatibility with IE 6+, Firefox 3+, etc...
You need to test your addon using multiple browsers to make sure it looks correct in all of them. If there are any inconsistencies then you need to work around them.
For CSS styling you will find compatibility charts online. For example:
These days I don't test much as modern browsers will generally be the same or very similar with rendering. Only problems will come with missing a closing tag and each browser handles it differently.
One that can show multiple versions side by side?
Also support for how thing will render on mobile browsers.
I looked at some of the online places but none of them are that appealing.