Section 29.15 Index Style
The Author's Guide explains how to create an index. The realization of the index is under the influence of the LaTeX imakeidx package, which results in certain stylistic choices, such as a two-column format. It also means that any options supported by this package could be easily implemented as the result of a feature request.
Also, there can be significant differences between how PreTeXt implements the index for HTML and how the imakeidx package creates an index for LaTeX. See Section 28.5 and [2].