Web design, like any other form of design, or art in general, is continuously changing. What was cool or hip today can look completely unappealing tomorrow, and that really is the beauty of design; the fact that it constantly makes you up your game, and create new, better works. Nobody could have expected the drastic changes in web design since the 90’s, since technology is such a rapidly evolving creature, and you can do things now -in design- that you would not have even dreamed of back in the 20th century.

Trends are the building blocks of culture. They appear and influence (for a while, at least) the way things are done, the way things look, or sound. Like Scott Cook said, major changes come about once a trend solidifies itself in culture, when it becomes more than just a passing fad. In web design, especially, trends are tremendously important, so it’s vital for you, as a web designer, to be ahead of the curve.