However, any user of any other CSS-capable browser written since 1998 can ignore or override any font size rule the author creates. In Internet Explorer 5 for Macintosh and Gecko-based browsers like Firefox, SeaMonkey, Netscape 6+, Compuserve 7+, and so forth, the user can simply use "Text Zoom" to resize text, even if it has been sized using pixels. In Opera, users can use "Page Zoom," which will resize both text and images. Finally, in IE/Win, motivated users can dig into the "Accessibility" options and check a box to "ignore font sizes specified on web pages". And for all these browsers, it is possible to write a user stylesheet that will override author styles. It is true that the vast majority of users are not aware of these options, but they are always there.
Thus it is not possible to always force font sizing with CSS, no matter what browser is in use. That is why it doesn't always work.
The reason it is not a good idea is that responsive font sizes, especially small ones, can be hard for those without 20/20 vision, and impossible for those with high resolution displays, to read.