Use this:
sidebarPanel(
...
, HTML("<I>Can</I> <em>this</em> <strong>happen</strong>?")
)
As an aside, you can even use renderText on the server side to build a full HTML output string, which can change, depending on your inputs. I often use this to post automated commentary (e.g. "The latest data release is y.... this is an increase of y, relative to the previous release...").