font size manipulation via end-user's browser controls

Hello,

Is the end user able to manipulate the font size of text that is
generated with CSS via browser preferences or browser font size
controls? Is there an instruction that enables or disables this feature?

If so, what is the default? Does anyone have a few URLs where I can look

at some examples of CCS and the font size attribute?  Any web sites
utilizing CSS, where I could test this out, would be especially helpful.

Thank you in advance,
Joshua