bug with my blog : how to fix it ?

hello, i have troubles with my blog, and i fail to fix it, can you help me ?

www.tailleurdimages.fr
wp = 2.9.2
atahualpa 3.4.6

the bug shows each time i write a new post : i can't write it correctly, it gets mad and change everything i do :
for example, if i erase one word, there is one blank line that appears somewhere in the post,
or suddenly a part or the whole post is written in very tiny letters (as in this post :
http://www.tailleurdimages.fr/ville-...-ville-ideale/
or it even erases phrases in an other side of the post so i have to be very carefully each time i jump a new line, or whatever !


so, this bug had shown first one month ago, and i fixed it by erasing the "addtoany" plugin that had been refreshed by a new version...

but it appeared again last week, and i did all these attempts without success :
- i suppressed all the plugins that where not compatible with my 2.9.2 wp version
- i desactivated the whole plugins and tried to write but the bug still works
- i tried the default theme (but with the 30 plugins) and it still works

i used this in order to fix the html errors : http://validator.w3.org/check?uri=ht...2F1.1#line-766
first i had 36 errors + 1 warning
now i just get 12 errors + 0 warning

i still get 5-6 errors with a youtube video (but i checked the code with comparing it to an other video and it looks the same, everything is ok)

and i noticed this error :


Line 766, Column 35: document type does not allow element "style" here
<style type="text/css" media="all">#avatar_footer { display: none; } /* Change …✉
The element named above was found in a context where it is not allowed. This could mean that you have incorrectly nested elements -- such as a "style" element in the "body" section instead of inside "head" -- or two elements that overlap (which is not allowed).

One common cause for this error is the use of XHTML syntax in HTML documents. Due to HTML's rules of implicitly closed elements, this error can create cascading effects. For instance, using XHTML's "self-closing" tags for "meta" and "link" in the "head" section of a HTML document may cause the parser to infer the end of the "head" section and the beginning of the "body" section (where "link" and "meta" are not allowed; hence the reported error


but i am not skilled to fix this, i tried to look in my dashboard, but i don't know what to do...

does anybody understand this and knows how to fix it ?
thanks for reading, have a nice day !