I'm on Windows 7, I can replicate this on FF 37.0.1 and IE11.0.9600.17728
Tables look fine in Chrome (Version 41.0.2272.89 m) (even though they're getting the problematic declaration)

batch.css:24 on Atlassian Stash v3.5.0

.markup table {
    border-collapse: collapse;
    margin: 20px 0px;
    width: 0px; /* <=== This one right here seems to be the problem child */
}

Here's a screenshot from FF: