Analyze the Reading Order
Reading order analysis performs a detailed reading order analysis of the page to determine whether reading order issues exist that violate the WCAG 2.0 1.3.2 Success Criteria, or that violate the Section 508 reading order rule for reading order without CSS.
The purpose of this check is to ensure that the content is presented to screen reader users and keyboard users in the proper order. Screen readers ignore the visual position of elements, relying on the DOM (Document Object Model) for the reading order of the content. Some CSS methods -- such as float, absolute positioning and relative positioning -- can modify the visual reading order without modifying the underlying DOM reading order, creating a mismatch that can be confusing to people who can't see the visual reading order. In some cases the content may not make sense at all when read out of order.
FireEyes briefly turns the style sheets off, then turns them back on, and compares the DOM order to the visual reading order for any discrepancies.