The richtext renderer consists of two elements, <rt> for richtext block and <p> as a text format block. Every text starting with <rt> in multiline textarea can display richtexts.
A richtext block holds many <p> format blocks, and can display one or many images. At least one richtext block and one format block is needed for displaying any stlyed text, so the minimum you need would be:
<rt text-align=center><p font-face=Verdana>Hello World</p></rt>
A format block is used to style text. Unlike in previous versions you can use many text styles on the same line. If you want yellow text in Verdana use the following:
<p font-face=Verdana font-color=#FF0000>This is red text</p>
Underlined heading and normal text in next line:
<rt><p font-decoration=underline font-size=14>Heading<br><br></p><p font-size=12>Normal text</p></rt>
Image line with some centered coloured text beneath it:
<rt image=img.png;img2.png;img3.png></rt><rt text-align=center><p font-color=#FF00FF>Colored text, </p><p>standard colored text.</p></rt>