Web browsers differences

Internet Explorer

• Debugging JavaScript on IE is a lot harder than on FF. The best is to install Firebug Lite. However, you must not use console.log() before the window is loaded (else you'll get a very annoying fatal error saying IE cannot open the page).
• Only use Prototype JS code or you'll run into troubles (mainly because all elements should be extended Prototype elements to ensure cross-browser behavior).
• IE caches *a lot*. In particular Ajax requests can be cached! Be sure to put some caching directives on the server side, else IE simply won't hit your server again. It'll use the cached version.
• IE 7 automatically adds margins to forms. You can override that behavior with CSS.
• IE 7 has a bug regarding elements with z-index (absolutely positioned elements with relatively positioned parents).
• IE does not properly support CSS comments; multiple CSS comments on one line, with an actual property / value set, confuses the poor piece of crap.
• IE seems to have a bug when multiple floated divs are present on a container, with their widths in percentage. If the percentage amounts to exactly one hundred (100%), then it will not render correctly. To fix this, you can just make the widths go only to 99% if possible.
• IE does not like absolute positioning on anchor elements or images; but you can usually fix it by wrapping in a div or span. Apparently for IE it is better to give both top and left properties for absolutely positioned elements (eg, both coordinates).
• text-align: center can also cause misbehavior, usually you can fix it by only applying this property to a subelement.

Links

• A good link to IE 7 bugs.

Opera

• location.hash is bugged when the hash contains a question mark. Update: this seems to have been fixed.
• myForm.readAttribute("action") behaves differently in Opera, as it adds the hostname before (which is bad!). The bug was apparently fixed in Opera 9.50 (see this bug reference).
• Opera automatically adds margins to forms.
• Opera does not fire the "load" event on an newly created image, which has already been downloaded (eg, whose source corresponds to an already fetched element). This is probably a bug that hopefully will be fixed.