Although most modern browsers can zoom in on text that is sized using pixels, not all of them can scale text sized with pixels – it still varies depending on the browser.
For example, in IE8, you can easily zoom in on text without any issues. However, if you try to scale text using the official text size modifier (by going to page -> text size -> largest), it doesn't work properly.
- In IE8 - Zoom works, but scaling does not
- In Firefox 4, beta 12 - Zoom works, but scaling does not
- In Google Chrome 10 beta - Zoom works, but scaling does not
Many organizations I collaborate with use the official text size modifier as part of their user/group settings management. When setting up accounts for users, they often pre-configure it using this setting. Personally, I prefer using EMs because PX doesn't scale consistently across different platforms.
And a few words from the W3C:
Units: avoid absolute length units for screen display
- Avoid specifying font-size in pt or other absolute length units for screen stylesheets, as they render inconsistently and cannot be resized by the User Agent (e.g., browser). Instead, use relative length units like percent or em.
Reference: http://www.w3.org/QA/Tips/font-size