In HTML5, would px change sizes on different monitors?

Question

If I had a 200 pixel and a 20 pixel monitor, and I set an image to be 20px on my html5 file with my big monitor, would it shrink if I were to view the website on my smaller monitor or would it take up the while screen?

Answer 1

It will take up the whole screen but you can scroll down/right

Answer 2

Yes, you can. Just write in percent. Example: width="100%". Picture automatically use AutoScalling factor.