Unfortunately even the popular browsers do not make it apparent enough. The next step, I think, is to build a text to let the user know how to resize the text in all browsers. It will really help if browsers include toolbar buttons for resizing text by default.
Not an extension of this, but as part of my experiment to give up control, I have unset the fonts and font sizes for this blog. Which means that currently it is using those settings from your browser. I always felt that in spite of
font-families you could never ensure using the best font on the platform. The reason I am calling this an experiment is because I am not sure if this will work for all and for all web sites. But this does let you choose the font and font-size you like to use.