Some special characters are not directly returned by innerHTML. For instance, < is returned as '>'.
This fiddle runs down the entire list of HTML entities and sees which are not returned unencoded.
Highcharts calls innerHTML() on the element containing its chart to retrieve the generated SVG.
Unfortunately, many browsers will translate a literal UTF NO-BREAK SPACE (U+00A0) into ' ', which is not a valid XML entity and thus invalid in SVG.