After a lot of frantic remote desktop hops, I figured out why my page was showing up on FireFox, but not IE 6 or 7. It was a pretty frustrating struggle, trying to turn one of my best visitor-grabbing pages back on.

One lesson is that I shouldn't use FF for most of my browsing. But everyone already knows that. The more novel lesson is don't make your script tags self closing; always use an open tag and a close tag if you want anything below them on the page to show up.

I've got enough java functionality on a big page that it made sense to put this into an external js file. I thought I'd be clever and save a few bytes, so I tried this:

<script src="ImageSwap.js" />

at the top of the page. When I navigated to the page in IE, I would get the nav bar and page header, but no content. Worse yet, I've made a lot of changes since the last time I looked at the page using IE, and spent a lot of time looking for a div set to display:none, or something like that.

<script src="ImageSwap.js"></script>

On the same topic (<sarcasm>don't ya just love IE</sarcasm>).
If you write canonical JSP's (strict XML format) then this:

 <script src="ImageSwap.js"></script>

will get converted to this

 <script src="ImageSwap.js" />

by the JSP compiler. So, its required to put something in the tag to prevent the collapse

 <script src="ImageSwap.js">/* image swap */</script>

I know you are trying to save every byte but, if you want pages that pass XHTML validation, be sure to add the type attibute.

One lesson is that I shouldn't use FF for most of my browsing.

Nope, I have to disagree. IE is the LEAST compliant with CSS and if you're using IT to weigh how your page is going to look in FF, Safari, or Opera, you're going to have nothing but problems !

IN XHTML EVERY tag must be closed. If a tag doesn't normally have a end tag, like the <script></script> tag DOES, then and only then would you use the / -- as you do for an <img> tag: <img src="blahblah" alt="blah" />

Your error was in not using the PROPER end tag for the <script>.

For once, I have to disagree with you ladynred.

Closing an xhtml tag with <tag /> is prefectly correct in all case.
But as you pointed out, the problem lies in IE.

The fact it that ie will read this notation correctly only if he is not in "quircks mode", which is fairly hard to be with his "@#§§" standard compliance.

In my firm, we are using the tandem XML (generated by oracle pl/sql)+XSL to generate web pages, and this is an well known problem, and the reason for so much exception rules for ie that we don't even count them anymore...

Originally Posted by ForrestCroce

One lesson is that I shouldn't use FF for most of my browsing. But everyone already knows that.

Actually, everyone disagrees with that statement. The usage of Firefox globally is around 30% but, for more technical users its much higher. On my website around 70% of visitors use Firefox. Also, the majority of Mac and Linux users use FF.
The lesson is that you should test your pages more (with a variety of browsers)
FF/IE/Opera at the minimum. You should also try for Safari/Konqueror if possible.
Something also recommended by many, is to develop in strict XHTML and send the correct mime-type (application/xhtml+xml) for it (yes it breaks IE, but FF is smarter) in your dev/test environments. This will cause the page to break for the smallest of html errors, allowing you to catch them early. Then use the text/html mime-type on public servers to allow the browser to be more lenient.