>1/ How important is well formed HTML/XHTML in terms of a search
Clean code is very important to search engines. If your code is
not clean, the spiders may not read the code properly. If you
have too much excess code the spiders just have to go through it
all. Clean short coding is best.
>2/ Will my sites be penalised because they are in XHTML?
No, you will not be penalized.
>3/ Do you know if XHTML is backwards-compatible with older forms
I run Netscape 4.6 and haven't noticed any problems with XHTML.
>4/ Do you believe that browsers will really enforce this standard
>in the future?
I don't know if the borwsers will "enforce" these standards, but
they will support them. They will probably never enforce these
standards because most average personal page owners will not
learn XHTML. It is more advanced and people will not take the
time to learn it for their family pages.
XHTML is only a transitional language. This is to lead us to XML.