No this is not possible.
As you seen ScreenOverlays are images only. You may however be able to
find some software to render your html as a image?
Don't think you are actully asking about having it disappear
automatically as the user moves about, but if you are checkout the
region functionality.
Also note that a certain amount of the <description> tag can be shown
in the 'Places' panel, which might be enough for a short
introduction?