You have a rule
#content, .entry-content h1, .entry-content h2 .side-by-side { float: left; margin-right: 2px }
That floats the h1
I believe the rule was supposed to be
#content, .entry-content h1 .side-by-side, .entry-content h2 .side-by-side { float: left; margin-right: 2px }
When a rule has several selectors, you must make sure to edit each of them.