C-grade
C-grade is the base level of support, providing core content and functionality. It is sometimes called core support. Delivered via nothing more than semantic HTML, the content and experience is highly accessible, unenhanced by decoration or advanced functionality, and forward and backward compatible. Layers of style and behavior are omitted.
C-grade browsers are identified on a blacklist. Approximately 3% of our audience receives a C-grade experience.
Summary: C-grade browsers are identified, incapable, antiquated and rare. QA tests a sampling of C-grade browsers, and bugs are addressed with high priority.
A-grade
A-grade support is the highest support level. By taking full advantage of the powerful capabilities of modern web standards, the A-grade experience provides advanced functionality and visual fidelity.
A-grade browsers are identified on a whitelist. Approximately 96% of our audience enjoys an A-grade experience.
Summary: A-grade browsers are identified, capable, modern and common. QA tests all A-grade browsers, and bugs are addressed with high priority.
X-grade
X-grade provides support for unknown, fringe or rare browsers. Browsers receiving X-grade support are assumed to be capable. (If a browser is shown to be incapable — if it chokes on modern methodologies and its user would be better served without decoration or functionality — then it is considered a C-grade browser.)
X-grade browsers include all browsers not on the C-grade blacklist or the A-grade whitelist. Approximately 1% of our audience receives the X-grade experience.
Summary: X-grade browsers are generally unknown, assumed to be capable, modern, and rare or fringe. QA does not test, and bugs are not opened against X-grade browsers.
The Relationship Between A- and X-grade Support
A bit more on the relationship between A and X grade browsers: One unexpected instance of X-grade is a newly-released version of an A-grade browser. Since thorough QA testing is an A-grade requirement, a brand-new (and therefore untested) browser does not qualify as an A-grade browser. This example highlights a strength of the Graded Browser Support approach. The only practical difference between A and X-grade browsers is that QA actively tests against A-grade browsers.
Unlike the C-grade, which receives only HTML, X-grade receives everything that A-grade does. Though a brand-new browser might be characterized initially as a X-grade browser, we give its users every chance to have the same experience as A-grade browsers.
