Discussion of the issue is available in Issue 77. To file comments on this proposal, please raise new issues for each discrete comment in GitHub.
Content can be zoomed to an equivalent width of 320 CSS pixels without loss of content or functionality, and without requiring scrolling on more than one axis except for parts of the content which require two-dimensional layout for usage or meaning.
320 CSS pixels is equivalent to a starting viewport width of 1280 CSS pixels wide at 400% zoom. For web pages which are designed to scroll horizontally, the 320px should be taken as the height rather than width.
Examples of content which require two-dimensional layout are images, maps, diagrams, video, games, presentations, data tables, and interfaces where it is necessary to keep toolbars in view while manipulating content.