Inserting a single-cell table with top border is a common workaround for the missing 'horizontal line' functionality. 

Exporting such table to PDF causes wrong result if the intercepting vertical borders are of type None and have smaller thickness.

Attached image outlines the issue.

Workaround: Set the same thickness to the noBorder and to the visible border:
var t = new Telerik.Windows.Documents.Model.Table(1, 1);\
var noBorder = new Telerik.Windows.Documents.Model.Border(3, BorderStyle.None, Colors.Black);
t.Borders = new TableBorders(noBorder, new Telerik.Windows.Documents.Model.Border(3, BorderStyle.Single, Colors.Black), noBorder, noBorder);


Steps to reproduce:
1. Insert table with the following code:

            var t = new Telerik.Windows.Documents.Model.Table(1, 1);
            var noBorder = new Telerik.Windows.Documents.Model.Border(BorderStyle.None);
            t.Borders = new TableBorders(noBorder, new Telerik.Windows.Documents.Model.Border(3, BorderStyle.Single, Colors.Black), noBorder, noBorder);

            this.radRichTextBox.InsertTable(t);
2. Export to PDF

Import the type of the <ul> and <ol> tag, e.g. <ul type="disc"> or <ul type=circle>. Supported values are disc, circle, square.

If there is sequence with more than one font info which is not declared in a separate group, they all are concatenated and recorded in the imported fonts as a single one with id from the last one. Here is such a problematic font table group:

{\\fonttbl\\f0\\froman\\fcharset0 Times New Roman;\\f1\\froman\\fcharset0 Times New Roman;\\f2\\froman\\fcharset0 Times New Roman;\\f3\\froman\\fcharset0 Times New Roman;\\f4\\froman\\fcharset0 Times New Roman;\\f5\\froman\\fcharset0 Times New Roman;}

If RadRichTextBox contains a table and is placed in a window and then the window is resized horizontally, the table border are moved unexpectedly and change their place relative to the content of the documen. If the window is maximized and/or restored, some of the borders disappear.

Workaround: Execute the following code snippet in window's or RadRichTextBox' SizeChanged event handler:
if (this.MyRadRichTextBox.ActiveEditorPresenter != null)
{
    this.MyRadRichTextBox.ActiveEditorPresenter.RecreateUI();
}

FloatingUIContainers are not cleared from the UI when they are deleted.

Workaround: Recreate UI using the following code:
this.radRichTextBox.ActiveEditorPresenter.RecreateUI();

Add support for math type equations.

When the user inserts a table in right-to-left mode, there is no ability to resize its rows/columns. The Table Properties dialog or the Document Ruler still can be used to resize the rows/columns of the Table.

When the document is in Web layout mode (with RadDocument.LayoutMode = "Flow"), and contains multiple sections with comments inside them, the comments are visualized on unexpected places outside of the comments pane, or one over another, hiding other comments.

Command that converts currently selected text to sentence case, lowercase, uppercase, capitalizes each word, or toggles case, while preserving the styling. A sample demonstrating how to add a custom implementation of a similar command is attached.

Include import and export in open XML - (<w:caps /> property in run properties) and in HTML (text-transform property).

A custom list has numbered list, which refer style and the style refer numbered list. If the second numbered list is not imported before importing the first the import will fail.

ArgumentException ("referenced document list does not exist."); is thrown and handled internally. It is caused by linked list styles. The style which refers other is imported before the referenced style. The RichTextBox expects that referred styles are imported before the styles that refer them.

Available in R3 2018 Official Release Version.

When floating image is added to empty table cell, it is positioned in a way that overlaps the table border. 

Implement support for exact table row height option. By using this option the text should be clipped when the table row height is lower than the height needed for the text to render. 
The current implementation supports only the at least option.

When floating image is all of the following: 
- is anchored to paragraph inside a table with negative offset.
- is with text wrapping "Behind text" or "In front of text"
it should be positioned outside of the table. Currently the image is always positioned fully inside the table.

Add support for importing/exporting macro-enabled document (*.dotm), while preserving the macro content.

Adding support for exporting font style as font tag. Android's TextView for example support styling only using tags.

When importing image with invalid ur/base64 encoded data the document won't be opened correctly and an exception will be thrown. Load default image instead of throwing exception and failing to open the document.

In paged layout mode, the caret becomes invisible when the document is scrolled to other page, so that the page with the caret goes out of the view port.

This behavior is also observed when editing, for example when inserting page break or section break.

Currently invalid attribute values, including invalid or empty values in CSS attributes in the 'style' attribute, could cause importing property with incorrect value, or NullReferenceException (the document cannot be imported).
Examples:
----------------------------------------------------------
<table width=”*”>
	<tr>
		<td>More Random Text</td>
	</tr>
</table>

The table is imported with 0 width. Instead the width should not be set.
----------------------------------------------------------
<table style="font-size:;">
	<tr>
		<td>More Random Text</td>
	</tr>
</table>

NullReferenceException is thrown.
----------------------------------------------------------

Workaround: replace the invalid values before import:
string importString = html.Replace("font-style: ;", "");