The ~ is a special character in Hyperlatex, and is replaced by
the HTML-tag for "non-breakable
space".
As we saw before, you can typeset the ~ character by typing
\~{}. This is also the way to go if you need the ~ in an
argument to an HTML command that is processed by Hyperlatex, such as
in the URL-argument of \xlink.
You can also use the \mbox command. It is implemented by replacing
all sequences of white space in the argument by a single
~. Obviously, this restricts what you can use in the
argument. (Better don't use any math mode material in the argument.)