Fw almost does it right. The correct way I think should be to export with the option File->HTML Setup->Document specific->Export multiple... selected. (There is a little caveat here: If you change the HTML Setup for a page, and start exporting with a other page selected it will use the HTML setup for that specific page, and will use that for ALL the pages in the PNG. Not a problem if you are aware of it, but it takes some time and a few wrong exports.). But this will not work right away.
Now it becomes somewhat fuzzy (sorry):
The problem is that there will be a number of files generated but the links in them are wrong, the most important are the page x_y.htm (with 2 buttons there will be only 4 files (page 1_1, apage 1_2, apage 2_1, page 2_2), with 3 there will be 9 etc.). whenever x equals y (page 1_1 and page 2_2) the page is useful. The only problem then is that the links in these files are wrong. (especially, the link in page 2_2 when i click the button for page 1 should go to page 1_1, but instead links to page 2_1 ).
So there are two workarounds:
1) after every export change the links in the page x_x files with a search and replace tool
2) Or for every file page x_y with x unequal y create a redirect html file which redirects to page y_y and copy that over the exported files after every export.
Or just ignore the problem
Maybe there are other solutions but I donot see them.
Thx for the advice. I''ve submitted this.