Fatal's Journal

Opera
Opera is an independent browser with a rather small market share (around 0.5%). It managed to survive—and even thrive—through the Browser Wars, without a large and prosperous corporation behind it. This achievement is not to be despised.
When it comes to JavaScript, Opera has always been slightly behind the other browsers. During the Browser Wars, it didn’t implement DHTML (i.e., the Microsoft and Netscape proprietary DOMs), which in hindsight was an excellent decision.
Although it has largely caught up with the other browsers, it remains the most difficult browser to develop scripts for. It has share of bugs, and its market share is so tiny that the temptation to just forget about it when you encounter a bug is difficult to resist.

Day-to-day development
When I create a script, I constantly test every line I write in Explorer Windows and Mozilla. Explorer contains quite a few peculiarities that should be addressed immediately, while Mozilla is an excellent representative of W3C DOM-compliant browsers.
When I finish a script module, I start Safari and Opera (and occasionally Explorer Mac, iCab, and Konqueror) to check for problems. If I find any, I try to solve them, but the amount of time and trouble I take depends on the browser in question.
Essentially I’m willing to take a lot of time for Safari, but not for any of the other browsers. Opera bugs are either easy or impossible to solve, Explorer Mac is too old, and iCab and Konqueror have too little market share. Agree or disagree with my exact browser lineup, but I advise you to make decisions or risk total insanity.
Do not under any circumstance write a script for one browser first and add support for the other browser “later on”. This is the fastest way to hell. It’s far better to solve nasty incompatibilities at the start of your project than at the end.