Thank you for your quick answer Ray.
I rebuilt a new site with just one blank php page generated by DW CS3
(only with CSS for builder)
Here are the requested information :
Choose a form > "contact" / "Appointment contact"
Choose a design > "cool"
Select for method > "post"
This time, I just had browsers compatibility errors (same quantity as before > see attached jpg file).
Jeffrey Zeldman's Web Standard Advisor is ok. I am glad about this point. I will have a look tomorrow to see why I had so many errors before...
So the remaining problem is the lack of compatibility with the main browsers (if e.g. IE6 and 7 !)...> test your forms in the DW tab "results" (F7), tab "chech the compatibility with the browsers (my english translation of french tab names).