A quick checklist.. (apologies for it ignoring some of which you stated you've already tried)
- Does the problem appear in both firefox and chrome? If only chrome it's probably a
display: block;
missing somewhere. - Inspect the element with firefox or chrome to see if wordpress or your theme is inserting any html/css. It may be inserting pre-styled
ul
li
orp
tags which you will need to style. - Make use of !important to override any css that may be set by wordpress core styles. Gallery styling, for example, is still set in core.
!important example usage (with some ideas as to how your theme or wordpress styling could be affecting it from somewhere)
display: block !important;
margin: 0px !important;
padding: 0px !important;
height: 100% !important;
max-height: 100% !important;
vertical-align: top !important;
font-size: 0 !important;
You can also try adding this filter to your functions.php file and then play with the gallery css.
add_filter( 'use_default_gallery_style', '__return_false' );
Here's a tutorial on styling galleries - http://theme.fm/2011/06/how-to-style-your-wordpress-gallery-43/
There are other filters to remove the auto style inserts by wordpress if you find those are the problem (as it sounds to be)