Use the following code:
this.radLabel1.Font = new Font("Segoe UI", 11);
this.radLabel1.Text = "<html> Sample <strong> strong </strong> text to render. <br/> Please <u><strong>underline this here</strong></u> and make it strong.";
The underlined text is not aligned correctly. If you change the font size to 14, then the empty spaces between the words in the underlined text are replaced with "_" and they are visible.
Workaround: set the TextWrap property to false