There are standards for the browser companies to follow so that everybody can be on the same page when it comes to how the browsers interpret
HTML,
CSS and Javascript. Because Microsoft has so many of their web development tools tied into each other it makes them very reluctant to apply themselves to newer specifications. Explorer 7 made great strides where
CSS is concerned (before that they were like a decade behind mozilla on some aspects of standards compliance) and will likely be as close to compliant as anybody has been when it comes to
CSS on Explorer 8.
They aren't doing jack about their Javascript problems, however, so if you don't bother to test your
JS code, it's a pretty good bet it will break on the other browser, especially if you're getting into in-depth
DOM-scripting type stuff. When building a deeper ap for the web, you're pretty much building one for Explorer and another one for everybody else.
There's a reason a lot of web designers and developers aren't big fans of Microsoft.
The only way to deal with the differences between the browsers is to fork your code, build your own library full of functions and objects that fix common problems so you don't have to deal with them twice, or learn JQuery or some other popular
JS library where that's all been done for you.