JavaScript: Forgiveness by default or how the web really works

Jeff Atwood has a good post on one of my pet peeves. The web is full of JavaScript and HTML errors.

At home I barely notice it but at work where I need to have Script Debugging enabled in IE for testing purposes, I find that some sites are almost unusable as there are sometimes 2-3 or even unlimited loops of errors with modal dialogs to click and continue. Does MS thinks that web users can actually debug other peoples websites?

"I quickly realized that the web is full of JavaScript errors. You can barely click through three links before encountering a JavaScript error of one kind or another. Often they come in pairs, triplets, sometimes dozens of them. It's nearly impossible to navigate the web with JavaScript error notification enabled.

JavaScript errors are so pervasive, in fact, that it's easy to understand why IE demotes them to nearly invisible statusbar elements. If they didn't, nobody would be able to browse the web without getting notified to death..."

Why are web developers so lax that they can't even do basic testing on their sites in at least IE and Firefox, the two most prevalent browsers? Agreed that the development environment should be less forgiving, as compilers do in most languages. With JS being interpreted you need to catch most bugs at runtime. Still it doesn't take much to find 90% of the common bugs on malformed JS by just setting IE properly and running your page.