I would like to use css to add an effect so that a div has a width of 500px but only if the user is on a computer. If the user is on a mobile phone, tablet, iPad etc, it would have the predefined width. How would I accomplish this?
You are looking for media query, read more about it here. If you are curious knowing different view widths, you can refer to this:-
OR
You can use responsive units to set widths, like 100%
or 100vw
, know more about responsive units here.