It means that you'll most probably have to revise your last statement and check back your code for errors.
p.s.: I guess "element's ID" is not being used as a "variable name", but as what it really is: a declared public reference to it.
The element's ID can only be directly used as a declared public reference in a .html or .xhtml file. If it is used in a file with any other markup extension (.xml, .svg), then this means of referring to it is only supported by Internet Explorer.
...(.xml, .svg), then this means of referring to it is only supported by Internet Explorer.
well, that's my boy... //others should follow!
Anyway, for cross-browser compatibility, you can always try my underDot platform which is faster than "document.getElementbyId", and smokes any "$()" based functions while being the shortest lib in existence, (weighting less than 40 bytes); using a simple syntax such as: _.elementID; including wrong id definitions: _["element ID"] possibility;
alas, I would never bother to serve my content in other than *.htm anyway.