Los diferentes tipos de zoom en una página web

En Zoom, zoom, and zoom - The three types of browser (and CSS!) magnification se explica:

In brief: there is wide support for three different types of ‘zoom’ – available both to site visitors and (to some extent) CSS authors:

• Page zoom is the default with a handy keyboard shortcut, and roughly matches behavior of the CSS zoom property.
• Scale factor (or ‘pinch zoom’) was introduced by early versions of mobile Safari, and may only be available through trackpad or touch interfaces – roughly matching the behavior of the CSS scale transform.
• Text-only zoom is also provided by Firefox and Safari. While not directly available in CSS, the behavior is similar to changing default font size on a site that uses entirely relative text sizing with rem units.