[Orgmode] How do you use org for other formats

[zwz]

Hi,

I recently found the problem when I want to export the org file to
html and pdf. I guess there will be some people (who are also not so
familiar with the powerful org-mode) bothered by the same issue, that
is, html and pdf requires different org format sometimes.
 - I use $ y = x_1^2 + x_2^2 $ in org, it works for pdf, but not for html
 - "_" in text will introduce mess in pdf, not in html

I definitely want to keep only one org which I can export to other
formats correctly. I do not know if it is already implemented in
org-mode.

I just wondered if it is possible to introduce a abstract level (or
notations) so that org can translate it according to the target
format, just like how org treat the headlines (*).