As of 2019, the minimum window width for both Chrome and Safari browser windows is 500px. Which is might annoying, if your are developing a site with breakpoint at less than 500px.

window.open(
  'https://codec.trembl.org/939',
  'myDevWindow',   
  'width=400,height=400,resizable,scrollbars=yes,status=yes'
)

Example:

Observations:

In Chrome, the window becomes fully resizable, up to a 188 x 113px - which seems to be the absolute minimal window size.

In Safari, the window becomes resizable up to the specified size, the minimum size being 100px wide and 77px high.