If a client changes text size on a browser, what does the client expect to happen?

I'm assuming we're taking this as just one element of a UX strategy we've drawn up for the site in question... That given, we need to think about the userbase for this particular UX enhancement... Users with a visual impairment requiring larger text are likely (if using their own machine) to have already made modifications to their machine/browser so as to comfortably read text across all the sites they may visit - (who wants to change the text size every time they visit a site?) these changes are more likely to entail a lower screen resolution and/or larger monitor than a global browser setting - (particularly in browsers which simply resize text and break layouts). If your text is so small as to prove difficult to read for a user who has already made such changes, then it is too small - full stop. These assumptions made, I consider it important to consider that users browsing your site requiring text-magnification don't deserve to have your layout fall to pieces - they may be visually impaired (they may simply be fortunate enough to be viewing your site on a huge monitor at a ridiculously high resolution - like many web designers) - either way, they are your users, they understand aesthetics just the same as the rest of your users and they aren't going to appreciate your CSS layout falling to bits because the text doesn't fit inside it. So scale the whole site - it may introduce scrollbars, but it will also magnify navigation elements, form elements, images, styling cues etc etc - all those other elements key to your design which may not be delivered with text - and which may not make sense when the surrounding text is enlarged...

Dom, I whole heartedly agree with your comments above and have always built my sites with em's. However, your arguments above can mostly be satisfied using the page zoom options in browsers. Now that all modern browsers have page zoom by default I'm beginning to question the way I work. Thinking with my semantics hat on, surely the 'text' should scale when selecting text size options... and the 'page' should scale when selecting page zoom options.

Should we replicate page zooming in CSS? when the browser does it by default AND much better.

Both options are legitimate and, ideally, both should be available. It should be possible to "zoom" an entire site, or increase the size or some or all of the primary text within a constrained width to facilitate easier reading.