Browser zoom unit for accessibility [css-values-and-units]

Currently all browsers scale the size of a pixel in order to zoom into a page by default. As a result, a zoomed in viewport is indistinguishable from a narrower viewport. This may result in unintended layout shifts as media queries are triggered as well as problems scaling responsive typography to be compatible with SC 1.4.4.

CSS authors need a way to identify zoom level so that we can create affordances for accessibility issues that may arise as a result of how zoom is currently implemented.

Two possible solutions:

1. A CSS keyword value that is effectively a unitless scale factor and can be used in calc functions.

h1 {
  font-size: calc(6vw * zoom);
}

2. Setting a standard for browsers to zoom the root font size instead of the size of a pixel. This will enable people to have finite control over what scales using relative units. Scale factor can be discovered with calc(1rem / 16px).
   Note: I don’t expect this to be an acceptable solution as the web has tried this and moved away from it to create a more consistent experience as many websites don’t use relative units.

Start with the css-values-and-units context named in the issue and review its two proposed approaches for exposing browser zoom to CSS authors. No specific file or test is identified; done would require an agreed standard solution that addresses zoom-related layout and responsive typography concerns.