The introduction of these browsers is very significant as far as designers are concerned. It's as if somebody had pulled the rug out from under your feet. Sitting in a back room somewhere is a group of people called the W3C - World Wide Web Consortium. It is their mission to establish standards for how the Web works. They collect and compile ideas for new features and if they agree that they are good ideas, they are introduced as 'standards' with the hope that all the browser producers will adopt them and that any Web page that adheres to those standards will work in all browsers, or at least, not break.
Standards make everything easier, and safer for everybody.
There are many such examples and clearly, somebody thought that they were a good idea at the time - and somebody else didn't. But, very few of us would dream of producing Web pages for only one browser and are painfully aware of the heartaches and frustrations caused by having to make sure that our pages work in as many browsers and browser versions as possible. With pressure from groups such as WaSP , browser manufacturers are slowly, but slowly, being persuaded into complying with the W3C's standards with their newer products. MsIE 5 for Mac goes a long way to meeting those standards, but sadly, not the whole way. Netscape 6 has been developed with the intention of being fully compliant and the preview release looks very promising from that aspect.