HTML, CSS

Hi, thanks for taking your time to look at this challenge. I am practicing CSS and HTML as a newbie and hoping to get some feedback from our fellow web designers. In this challenge, I found a few questions to mind:

• When setting up the grid, should I use unit Pixel (px) or Percentage (%)? Both work fine but using percentages may move the context inside and mess up the align-items. Pixel is good and very constant but it doesn't feel responsive(?)
• I am using 2 screens: a 2k and a FullHD screen. I found that the size of the website is not the same on the two screens (Ex: too big on fullHD but small on 2k screen). Should I change them to the same resuluation or is there any standard that I need to take into consideration? Thank you!