This check flags when heading styles in the InDesign document have been applied out of sequence.
By marking paragraphs with a suitable export tag we add extra semantic meaning to the text of our publications.
The sequence that headings are applied is important for navigation, for example a screen reader user may choose to scan through a long section of text by only reading the headings and subheadings. Heading hierarchy is the term used to mean headings are applied in a predictable and logical way.
There are up to six levels of heading available in HTML and therefore in EPUB. From heading level 1 <h1> to heading level 6 <h6> .
When ACE, by DAISY reports on well made accessible documents it checks that heading hierarchy is applied in the correct sequence. For example a top level heading <h1> tag followed by paragraph <p> tags is to be expected but a heading level 3 <h3> tags should not be used unless it has been preceded by an <h2> heading (which itself has been preceded by a heading level 1 <h1> tag)
This GreenLight checks and flags whether heading are applied in sequence.
If headings are not applied in sequence then the user is taken to their location on the InDesign page and the text is highlighted text.
A dialog shows that a paragraph of text has an h3 tag but id preced by an h1 tag. The recommended method to manage and apply headings is to use Paragrah Style effectively.
More info on heading hierarchy from the W3C Web Accessibility Initiative (WAI)
