Mainly I use Firefox to work with and when having pics in articles they always fit into the Twenty Eleven layout.
Recently I used some pics of 590px width and Twenty Eleven always arranges that they appear reduced to about 550px width, perfectly fitting into the post area and only increase size when clicking on them (Fancybox for WordPress (version 2.7.5)).
Not so when using IE, Opera or Google Chrome. They just display things as they are and even enter the sidebar. Same with youtube embedded videos, they are often 560px width what is too big.
Question is, if Firefox understands the script to shrink pics and videos to the right size, why not all those other browsers? After all, what does cross-browser compatibility mean?