If you want to end up with cross-browser HTML then you really don't want to start out in PowerPoint 
One thing you could try is importing the presentation into OpenOffice and then using the export to HTML functionality of that - probably the HTML will still be a bit of a mess, but it's far more likely to work cross-browser.
If just having it viewable cross-platform would be enough you could also try exporting as a PDF, OpenOffice will do this too or try FoxIT PDF Creator .
