alt131 - 4:44 am on May 20, 2011 (gmt 0)
Hi BeLlSiBuB [webmasterworld.com], and welcome to WebmasterWorld :)
I'm not sure about the "crooked border" as browsers draw straight lines, and I couldn't reproduce the issue. Always useful to validate [validator.w3.org] when code is not behaving as desired.
Note you have set border-width:0; That specifies a zero-width border, so there should be no border showing at all.
In terms of changes from "solid" to "thin" causing the borders to vanish, make sure you have the correct property/value match. For example
border-width [w3.org] takes thin/thick/medium and a length (like 1px)
border-style [w3.org] takes solid/dashed etc.
So for eg, border-width:solid; has no meaning, which could be why the border "disappears"
firebug will reveal the styles being applied to the elements. There is a picture and explanation of that "part" in the html development/view source live [getfirebug.com] section of the help documentation