Websites should display perfectly no matter what device or screen resolution is used.
In the early days of the web, web pages only had to look good in two or three screen sizes and resolutions. Then, when bigger monitors with higher resolutions became available, the existing websites looked out of date.
Then, more resolutions became available, rendering new designs yet again outdated. In today's digital landscape, a plethora of mobile phones, tablets, monitors, and laptops, has resulted in a huge number of screen resolutions that a website has to size to.