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How do we know what the default px is? Or is it always 16px?
It depends on a few things. Basically percentages and em's are both scalable, meaning they'll adjust depending on the size of document. They're both responsive.
This blog gives a great explanation of the differences. He gives a slightly biased answer for his preference for working with percentages. I wouldn't get to hung up on trying to force yourself to use em's in your work but understanding the theory behind them is important because you will come across them in other people's work.
Thanks for both your answers! I get the theory behind ems.. just wasn't sure if 16px was the default size of text unless otherwise stated..
for example.. would there every be a situation where a browser defaults the text size to 12px, instead of 16px?