When web designers switch from focusing on a single browser (usually Internet Explorer) to developing cross-browser sites (usually adding Firefox, sometimes Opera or Safari, ideally all three), they often find that things don’t work as expected in the “new” browser. This can be for a number of reasons, including:
- Bugs or “missing” features in the new browser (whether incomplete support in the new browser, or proprietary features in the familiar browser).
- Broken code on the website being handled differently.
- Different defaults where behavior isn’t well-defined in the specifications.
A big problem is that when you get into the code, a lot of pages aren’t as specific as the authors think they are. When you write code and test it on one browser, you’re not testing that the code is correct, you’re testing that that browser makes the same assumptions you do.
It’s like ordering pizza.
No, really. Let’s say Internet Explorer specializes in Chicago-style pizza, with a thick, chewy crust. And let’s say Firefox specializes in New York-style pizza, with a thin crust. But each can make the other style of pizza on request.
So you call up Internet Explorer and ask for pizza. They deliver you Chicago pizza, and if that’s what you wanted, you figure your order is fine. If you actually wanted New York style, you make sure that next time, you tell them you want that style of pizza.
But let’s say you like Chicago pizza. You get used to calling up IE and just asking for “pizza,” until one day you’re busy, and ask your roommate to order it. He likes to get his pizza from Firefox, so he calls them up, asks for “pizza,” and you get New York style. That’s not what you wanted. Obviously, Firefox pizza is inferior, because they got the order wrong! Well, no, it’s not, and no, they didn’t. They delivered what they were asked for. If you’d told your roommate to ask for Chicago style, Firefox would have been perfectly happy to deliver that style of pizza.
The moral of the story: always be specific with your code. Make sure it’s asking for what you think it’s asking for (validation helps here). And if something doesn’t do what you expect, make sure you didn’t leave that expectation out of your order.
See also: No, Internet Explorer did not handle it properly
(Expanded from a comment I posted at Mozillazine.)