checking for supported character set

[Ronald Fischer]

Consider the Unicode character

Š

I found that it displays correctly on most installation I tested, but sometimes
a browser is not able to display it at all - perhaps because it has not installed
the correct character set, or because it is a very old browser.

Ideally I would like to write my (static) HTML page so that the browser displays
the correct character for the aforementioned code, if possible, but displays the
letter S otherwise.

I guess I'm asking too much here - but maybe someone can suggest how I can
get closer to what I would like to achieve?

Ronald