It's not just that it had quirks, it's that your feedback loop was longer in IE compared to Firefox with Firebug and Chrome. IE 7 did not, to my recollection, have a way to open developer tools and tweak CSS values until it looked right, or even to see what the effective CSS on an element was. So you had to save and refresh. Sure, that's not a super long cycle time compared to, say, compiling a binary and flashing to a ROM, but it's longer than the other browsers I was working with at the time.