What should I wear in Europe?

I have never been outside the US or Canada. I am going to visit The UK and The Netherlands. As an American, I have heard that people dress more formally in Europe. Is this true? Is there some outfits that I should leave home, and others that I should take or invest in to take? I particularly would like to hear from those who have been in both North America and Europe. Thanks.

What should I wear in Europe?
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