HTML heading elements (<h1>-<h6>) are imported with the default heading styles (Heading 1 - Heading 6) for RadFlowDocument, which is unexpected, as the browsers and MS Word import them with different styling. For example, <h1> is imported as Heading 1 with font the following properties:
Font: Cambria
Font color: Accent 1
Spacing before: 14 pt
Spacing after: 14 pt
While MS Word imports it as:
Font: Times New Roman
Font color: not set (black)
Character spacing: Kern at 18 pt
Spacing before: Auto
Spacing after: Auto