In addition to being able to control the general zoom of the page, there are users who may need to be able to control only the zoom of the text. And by doing so they shouldn't lose any content or functionality, or create overlaps that didn't exist.

The minimum limit for this zoom is set at 200% by the WCAG:

The working group feels that 200% is a reasonable accommodation that can support a wide range of designs and layouts.

For this, it is recommended to use the text sizes in relative units, with a fallback to pixels. Although browsers are increasingly capable of supporting text zoom well even if it is defined in pixels, today it is more reliable to do it with em or rem, which is somewhat more comfortable to use.

In this interesting post from The A11Y Project you can find information on how to perform the tests to see if our page meets this criteria.