我做到了。将元素转换为div，然后通过Role获取标记名。

var div = element as Div;
if (div.GetAccessibilityProperties().GetRole() == "H1") title = true;

和额外的helper来从元素中获取内容，以填充TOC：

private static string GetContent(IElement element)
{
    var builder = new StringBuilder();
    if (element is Text) builder.Append(((Text)element).GetText());
    if (element is IAbstractElement)
        foreach (var child in (element as IAbstractElement).GetChildren())
            builder.Append(GetContent(child));
    return builder.ToString();
}