Client devices
Web browsers are available on a range of devices these days, and with those devices come a range of screen resolutions.
A website should be designed in such a way that all content is available to any user regardless of device even if the path to access the data varies. For example, a mobile website might look and behave differently to the version you see on a desktop computer, but the available content is the same.
You also need to consider how people with disabilities can access your site. A blind person may not be able to see an image on the screen, but they should still be told what the image contained or what the point of it was.
Operating systems
Operating systems have their own way of displaying websites. This can affect the number and depth of colours that will be displayed on screen.
It can also affect the look of text, for example, a Mac, has different text-aliasing (smoothness) than a Windows computer. Not all fonts will be supported across all operating systems. This is why in CSS you state a list of different fonts to try. For example the Arial font on Windows is very similar (but not identical) to the Helvetica font on iOS.
Forms might also look different depending on the operating systems and browser being used.
