1. Using rem values makes it easier to scale a site via media queries since rem is based on the root font size. There are other alternatives to consider though.
  2. Seconds are a bit easier to mentally parse than milliseconds although this can largely come down to user preference.
  3. If you're only adding a left margin, it's clearer if you do margin-left: 1rem over margin: 0 0 0 1rem. It also erases the chance of getting unnecessarily overwritten later.